Light-emitting element, light-emitting device, electronic device, and lighting device

ABSTRACT

A light-emitting element that includes a fluorescent material and has a high emission efficiency is provided. A light-emitting element in which a delayed fluorescence component due to TTA accounts for a high proportion of emissive components is provided. A novel light-emitting device with a high emission efficiency and a low power consumption is provided. A light-emitting element includes an anode, a cathode, and an EL layer. The EL layer includes a light-emitting layer including a host material and an electron-transport layer including a first material in contact with the light-emitting layer. The LUMO level of the first material is lower than that of the host material. The proportion of a delayed fluorescence component due to TTA is greater than or equal to 10 percent of the light emission from the EL layer. The proportion of the delayed fluorescence component due to TTA may be greater than or equal to 15 percent of the light emission.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/791,231, filed Feb. 14, 2020, now allowed, which is a continuation ofU.S. application Ser. No. 15/985,748, filed May 22, 2018, now U.S. Pat.No. 10,573,837, which is a continuation of U.S. application Ser. No.15/362,932, filed Nov. 29, 2016, now U.S. Pat. No. 9,985,233, whichclaims the benefit of foreign priority applications filed in Japan asSerial No. 2015-234485 on Dec. 1, 2015, and Serial No. 2016-051071 onMar. 15, 2016, all of which are incorporated by reference.

TECHNICAL FIELD

One embodiment of the present invention relates to a light-emittingelement, a display module, a lighting module, a display device, alight-emitting device, an electronic device, and a lighting device. Notethat one embodiment of the present invention is not limited to the abovetechnical field. The technical field of one embodiment of the inventiondisclosed in this specification and the like relates to an object, amethod, or a manufacturing method. Furthermore, one embodiment of thepresent invention relates to a process, a machine, manufacture, or acomposition of matter. In particular, examples of the technical field ofone embodiment of the present invention disclosed in this specificationinclude a semiconductor device, a display device, a liquid crystaldisplay device, a light-emitting device, a lighting device, a powerstorage device, a memory device, an imaging device, a method for drivingany of them, and a method for manufacturing any of them.

BACKGROUND ART

In recent years, light-emitting elements using electroluminescence (EL)have been actively researched and developed. In a basic structure ofsuch a light-emitting element, a layer containing a light-emittingmaterial (an EL layer) is interposed between a pair of electrodes. Byapplying a voltage between the pair of electrodes of this element, lightemission from the light-emitting material can be obtained.

Since the above light-emitting element is of a self-luminous type, adisplay device using this light-emitting element has advantages such ashigh visibility, no necessity of a backlight, and low power consumption.Furthermore, the light-emitting element is also effective in reducingthe thickness and weight of the display device and increasing theresponse speed thereof.

In a light-emitting element (e.g., an organic EL element) including anEL layer that contains an organic light-emitting material and isprovided between a pair of electrodes, application of a voltage betweenthe pair of electrodes causes injection of electrons from a cathode andholes from an anode into the EL layer having a light-emitting propertyand thus a current flows. Then, the injected electrons and holesrecombine, so that the organic material having a light-emitting propertyis brought into an excited state to provide light emission.

The excited state formed by an organic material can be a singlet excitedstate (S*) or a triplet excited state (T*). Light emission from thesinglet excited state is referred to as fluorescence, and light emissionfrom the triplet excited state is referred to as phosphorescence. Thestatistical generation ratio of S* to T* in the light-emitting elementis 1:3. In other words, a light-emitting element including a materialemitting phosphorescence has a higher light emission efficiency than alight-emitting element including a material emitting fluorescence.Therefore, light-emitting elements including phosphorescent materialscapable of converting a triplet excited state into light emission havebeen actively developed in recent years.

Among the light-emitting elements including phosphorescent materials, alight-emitting element that emits blue light has not been put intopractical use yet because it is difficult to develop a stable materialhaving a high triplet excitation energy level. For this reason, a morestable fluorescent material has been developed for a light-emittingelement that emits blue light and a technique of increasing the emissionefficiency of the light-emitting element including a fluorescentmaterial has been searched.

As an emission mechanism capable of converting part of a triplet excitedstate into light emission, triplet-triplet annihilation (TTA) is known.The TTA refers to a process in which, when two triplet excitons approacheach other, excitation energy is transferred and spin angular momentumis exchanged to form a singlet exciton.

As compounds in which TTA occurs, anthracene compounds are known.Non-Patent Document 1 discloses that the use of an anthracene compoundas a host material in a light-emitting element that emits blue lightachieves an external quantum efficiency exceeding 10%. It also disclosesthat the proportion of a delayed fluorescence component due to TTA inthe anthracene compound is approximately 10% of emissive components ofthe light-emitting element.

Furthermore, tetracene compounds are known as compounds having a highproportion of a delayed fluorescence component due to TTA. Non-PatentDocument 2 discloses that the delayed fluorescence component due to TTAin light emission from a tetracene compound accounts for a higherproportion than that for an anthracene compound.

Note that when TTA occurs, the lifetime of a fluorescent materialsignificantly increases (delayed fluorescence is generated) as comparedto the case where TTA does not occur. The delayed fluorescence in alight-emitting element can be confirmed by observing the attenuation oflight emission after the steady injection of carriers is stopped at acertain point of time. Note that in that case, the spectrum of delayedfluorescence overlaps with the emission spectrum during the steadyinjection of carriers.

REFERENCE Non-Patent Document

[Non-Patent Document 1] Tsunenori SUZUKI and six others, JapaneseJournal of Applied Physics, vol. 53, 052102 (2014)[Non-Patent Document 2] D. Y. Kondakov and three others, Journal ofApplied Physics, vol. 106, 124510 (2009)

DISCLOSURE OF INVENTION

What is essential to improve the emission efficiency of a light-emittingelement including a fluorescent material is that the energy of tripletexcitons, which do not contribute to light emission, is converted intothe energy of singlet excitons with light-emitting properties and theconversion efficiency is increased. In other words, it is important toconvert the energy of triplet excitons into the energy of singletexcitons by TTA; in particular, the proportion of a delayed fluorescencecomponent due to TTA in the emissive components of the light-emittingelement should be increased. This is because an increased proportion ofthe delayed fluorescence component due to TTA means an increase in theproduction rate of singlet excitons with light-emitting properties.

In view of the above, an object of one embodiment of the presentinvention is to provide a light-emitting element that includes afluorescent material and has a high emission efficiency. Another objectof one embodiment of the present invention is to provide alight-emitting element in which a delayed fluorescence component due toTTA accounts for a high proportion of emissive components. Anotherobject of one embodiment of the present invention is to provide a novellight-emitting device with a high emission efficiency and a low powerconsumption. Another object of one embodiment of the present inventionis to provide a novel display device.

Note that the description of the above objects does not preclude theexistence of other objects. One embodiment of the present invention doesnot necessarily achieve all the objects. Other objects are apparent fromand can be derived from the description of the specification and thelike.

One embodiment of the present invention is a light-emitting elementincluding an anode, a cathode, and an EL layer between the anode and thecathode. The EL layer includes a light-emitting layer and anelectron-transport layer in contact with the light-emitting layer. Thelight-emitting layer includes a host material. The electron-transportlayer includes a first material. The LUMO (Lowest Unoccupied MolecularOrbital) level of the first material is lower than that of the hostmaterial. The proportion of a delayed fluorescence component due totriplet-triplet annihilation is greater than or equal to 10% of theentire light emission from the EL layer.

One embodiment of the present invention is a light-emitting elementincluding an anode, a cathode, and an EL layer between the anode and thecathode. The EL layer includes a light-emitting layer and anelectron-transport layer in contact with the light-emitting layer. Thelight-emitting layer includes a host material. The electron-transportlayer includes a first material. The LUMO level of the first material islower than that of the host material by greater than or equal to 0.05eV. The proportion of a delayed fluorescence component due totriplet-triplet annihilation is greater than or equal to 10% of theentire light emission from the EL layer.

Note that in one embodiment of the present invention, the proportion ofthe delayed fluorescence component due to the triplet-tripletannihilation may be greater than or equal to 15% of the entire lightemission from the EL layer. The first material may be a substanceincluding a condensed heteroaromatic ring skeleton having a diazineskeleton or a triazine skeleton. The first material may be a substanceincluding a pyrazine skeleton or a pyrimidine skeleton. The tripletexcitation energy of the first material may be higher than that of asubstance that has the highest triplet excitation energy among thematerials contained in the light-emitting layer by greater than or equalto 0.2 eV.

One embodiment of the present invention may be a light-emitting elementincluding a hole-transport layer in contact with the light-emittinglayer. The hole-transport layer includes a second material. The LUMOlevel of the second material is higher than that of the host material.Alternatively, in the light-emitting element including thehole-transport layer in contact with the light-emitting layer, thehole-transport layer may include the second material and the tripletexcitation energy of the second material may be higher than that of asubstance that has the highest triplet excitation energy among thematerials contained in the light-emitting layer by greater than or equalto 0.2 eV.

One embodiment of the present invention may be a light-emitting elementincluding the light-emitting layer further containing a fluorescentmaterial. The triplet excitation energy of the fluorescent material maybe higher than that of the host material. The LUMO level of thefluorescent material may be higher than or equal to that of the hostmaterial. The light-emitting layer may emit blue light.

One embodiment of the present invention is a light-emitting deviceincluding the light-emitting element and a transistor or a substrate.Another embodiment of the present invention may be an electronic deviceincluding a sensor, an operation button, a speaker, or a microphone inaddition to the light-emitting device. Another embodiment of the presentinvention may be a lighting device including a housing in addition tothe light-emitting device.

According to one embodiment of the present invention, a light-emittingelement that includes a fluorescent material and has a high emissionefficiency can be provided. According to another embodiment of thepresent invention, a light-emitting element in which a delayedfluorescence component due to TTA accounts for a high proportion ofemissive components can be provided. According to another embodiment ofthe present invention, a novel light-emitting device with a highemission efficiency and a low power consumption can be provided.According to another embodiment of the present invention, a noveldisplay device can be provided.

Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot necessarily have all the effects. Other effects are apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are schematic cross-sectional views of a light-emittingelement of one embodiment of the present invention and a schematicdiagram illustrating the correlation of energy levels;

FIG. 2 shows an energy barrier and a recombination region;

FIGS. 3A to 3C show the components of a transition dipole moment;

FIGS. 4A and 4B are schematic diagrams illustrating a measurement methodof molecular orientation;

FIG. 5 is a schematic cross-sectional view of a light-emitting elementof one embodiment of the present invention;

FIGS. 6A and 6B are schematic cross-sectional views of light-emittingelements of one embodiment of the present invention;

FIGS. 7A and 7B are a schematic cross-sectional view of a light-emittingelement of one embodiment of the present invention and a schematicdiagram illustrating the correlation of energy levels;

FIGS. 8A and 8B are a schematic cross-sectional view of a light-emittingelement of one embodiment of the present invention and a schematicdiagram illustrating the correlation of energy levels;

FIGS. 9A and 9B are a block diagram and a circuit diagram illustrating adisplay device of one embodiment of the present invention;

FIGS. 10A and 10B are perspective views illustrating an example of atouch panel of one embodiment of the present invention;

FIGS. 11A to 11C are cross-sectional views illustrating examples of thedisplay device and the touch sensor of one embodiment of the presentinvention;

FIGS. 12A and 12B are cross-sectional views illustrating examples of atouch panel of one embodiment of the present invention;

FIGS. 13A and 13B are a block diagram and a timing chart of a touchsensor of one embodiment of the present invention;

FIG. 14 is a circuit diagram of a touch sensor of one embodiment of thepresent invention;

FIG. 15 is a perspective view illustrating a display module of oneembodiment of the present invention;

FIGS. 16A to 16G illustrate electronic devices of one embodiment of thepresent invention;

FIG. 17 illustrates lighting devices of one embodiment of the presentinvention;

FIG. 18 illustrates a light-emitting element;

FIG. 19 shows the proportion of a delayed fluorescence component versusthe LUMO level of light-emitting elements 1 to 8;

FIG. 20 shows the external quantum efficiency versus the proportion of adelayed fluorescence component of the light-emitting elements 1 to 8;

FIG. 21 shows the current density-luminance characteristics of alight-emitting element 4-2;

FIG. 22 shows the voltage-luminance characteristics of thelight-emitting element 4-2;

FIG. 23 shows the luminance-current efficiency characteristics of thelight-emitting element 4-2;

FIG. 24 shows the voltage-current characteristics of the light-emittingelement 4-2;

FIG. 25 shows the luminance-external quantum efficiency characteristicsof the light-emitting element 4-2;

FIG. 26 shows the emission spectrum of the light-emitting element 4-2;

FIG. 27 shows an attenuation curve of transient fluorescence of thelight-emitting element 4-2;

FIG. 28 shows the reliability of the light-emitting element 4-2; and

FIG. 29 shows the angular dependence of the light-emitting element 9 andthe calculation result.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described.Note that it is easily understood by those skilled in the art that modesand details disclosed herein can be modified in various ways withoutdeparting from the spirit and scope of the present invention. Therefore,the present invention should not be interpreted as being limited to thedescription in the embodiments.

Note that in each drawing described in this specification, the size, thethickness, and the like of components such as an anode, an EL layer, anintermediate layer, and a cathode are exaggerated for clarity in somecases. Therefore, the sizes of the components are not limited to thesizes in the drawings and relative sizes between the components.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used for convenience and do not denote theorder of steps or the positional relation. Therefore, for example,description can be made even when “first” is replaced with “second” or“third”, as appropriate. In addition, the ordinal numbers in thisspecification and the like are not necessarily the same as those thatspecify one embodiment of the present invention.

In the structures of the present invention described in thisspecification and the like, the same portions or portions having similarfunctions are denoted by common reference numerals in differentdrawings, and the description of such portions is not repeated.Furthermore, the same hatching pattern is applied to portions havingsimilar functions, and the portions are not especially denoted byreference numerals in some cases.

In this specification, color is defined by three aspects of hue(corresponding to the wavelength of light of a single color), chroma(saturation, i.e., the degree to which it differs from white), and value(brightness, i.e., the intensity of light). In this specification, colormay be defined by only one of the above three aspects or two of theaspects which are selected arbitrarily. In this specification, adifference between two colors of light means a difference in at leastone of the above three aspects and also includes a difference in theshape of two spectra of light or in the distribution of the relativeintensity of the peaks in the spectra.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases, and the term “insulating film” can be changed into the term“insulating layer” in some cases.

In this specification and the like, a singlet excited state (S*) refersto a singlet state having excitation energy. Among the singlet excitedstates, the excited state having the lowest energy is referred to as alowest singlet excited state. A singlet excitation energy level means anenergy level in a singlet excited state. Among the singlet excitationenergy levels, the lowest excitation energy level is referred to as alowest singlet excitation energy (S1) level. Note that in thisspecification and the like, simple expressions “singlet excited state”and “singlet excitation energy level” mean the lowest singlet excitedstate and the S1 level, respectively, in some cases.

In this specification and the like, a triplet excited state (T*) refersto a triplet state having excitation energy. Among the triplet excitedstates, the excited state having the lowest energy is referred to as alowest triplet excited state. A triplet excitation energy level means anenergy level in a triplet excited state. Among the triplet excitationenergy levels, the lowest excitation energy level is referred to as alowest triplet excitation energy (T1) level. Note that in thisspecification and the like, simple expressions “triplet excited state”and “triplet excitation energy level” mean the lowest triplet excitedstate and the T1 level, respectively, in some cases.

In this specification and the like, a fluorescent material refers to amaterial that emits light in the visible light region when the singletexcited state relaxes to the ground state. A phosphorescent materialrefers to a material that emits light in the visible light region atroom temperature when the triplet excited state relaxes to the groundstate. That is, a phosphorescent material refers to a material that canconvert triplet excitation energy into visible light.

Note that in this specification and the like, “room temperature” refersto a temperature in a range of 0° C. to 40° C.

In this specification and the like, a wavelength range of blue refers toa wavelength range of greater than or equal to 400 nm and less than orequal to 550 nm, and blue light has at least one peak in that range inan emission spectrum.

Embodiment 1 <Structure Example of Light-Emitting Element>

First, a structure of a light-emitting element of one embodiment of thepresent invention will be described below with reference to FIGS. 1A to1C.

FIG. 1A is a schematic cross-sectional view of a light-emitting element150 of one embodiment of the present invention.

The light-emitting element 150 includes a pair of electrodes (anelectrode 101 and an electrode 102) and an EL layer 100 therebetween.The EL layer 100 includes at least a light-emitting layer 130. Note thatthe description in this embodiment is given assuming that the electrode101 and the electrode 102 of the pair of electrodes serve as an anodeand a cathode, respectively; however, they can be interchanged for thestructure of the light-emitting element 150.

The EL layer 100 illustrated in FIG. 1A includes functional layers inaddition to the light-emitting layer 130. The functional layers includea hole-injection layer 111, a hole-transport layer 112, anelectron-transport layer 118, and an electron-injection layer 119. Notethat the structure of the EL layer 100 is not limited to the structureillustrated in FIG. 1A, and a structure including at least one layerselected from the hole-injection layer 111, the hole-transport layer112, the electron-transport layer 118, and the electron-injection layer119 may be employed. Alternatively, the EL layer 100 may include afunctional layer which is capable of lowering a hole- orelectron-injection barrier, improving a hole- or electron-transportproperty, inhibiting transport of holes or electrons, or suppressing aquenching phenomenon by an electrode, for example.

FIG. 1B is a schematic cross-sectional view illustrating an example ofthe light-emitting layer 130 in FIG. 1A. The light-emitting layer 130 inFIG. 1B includes at least a host material 131 and a guest material 132.

The host material 131 preferably has a function of converting tripletexcitation energy into singlet excitation energy by causing TTA, so thatthe triplet excitation energy generated in the light-emitting layer 130can be partly converted into singlet excitation energy by TTA in thehost material 131. The singlet excitation energy generated by TTA can betransferred to the guest material 132 and extracted as fluorescence. Inorder to achieve this, the lowest singlet excitation energy (S1) levelof the host material 131 is preferably higher than the S1 level of theguest material 132. In addition, the lowest triplet excitation energy(T1) level of the host material 131 is preferably lower than the T1level of the guest material 132.

Note that the host material 131 may be composed of a single compound ora plurality of compounds. The guest material 132 may be a light-emittingorganic material, and the light-emitting organic material is preferablya material capable of emitting fluorescence (hereinafter also referredto as a fluorescent material). A structure in which a fluorescentmaterial is used as the guest material 132 will be described below. Theguest material 132 may be rephrased as the fluorescent material.

<Emission Mechanism of Light-Emitting Element>

First, the emission mechanism of the light-emitting element 150 isdescribed below.

In the light-emitting element 150 of one embodiment of the presentinvention, voltage application between the pair of electrodes (theelectrodes 101 and 102) causes electrons and holes to be injected fromthe cathode and the anode, respectively, into the EL layer 100 and thuscurrent flows. By recombination of the injected electrons and holes,excitons are formed. The ratio of singlet excitons to triplet excitonswhich are generated by carrier recombination is approximately 1:3according to the statistically obtained probability. Hence, theprobability of formation of singlet excitons is 25%.

Note that the exciton refers to a carrier (electron and hole) pair.Since excitons have excitation energy, a material where excitons aregenerated is brought into an excited state.

Through the following two processes, singlet excitons are formed in theEL layer 100 and light emission from the guest material 132 can beobtained:

(α) Direct formation process; and(β) TTA process.

<<(α) Direct Formation Process>>

Described first is the case where carriers (electrons and holes)recombine in the light-emitting layer 130 included in the EL layer 100to form a singlet exciton.

When the carriers recombine in the host material 131, excitons areformed to bring the host material 131 into an excited state (a singletexcited state or a triplet excited state). At this time, in the casewhere the excited state of the host material 131 is a singlet excitedstate, singlet excitation energy transfers from the S1 level of the hostmaterial 131 to the S1 level of the guest material 132, thereby formingthe singlet excited state of the guest material 132. Note that the casewhere the excited state of the host material 131 is a triplet excitedstate is described later in (β) TTA process.

When the carriers recombine in the guest material 132, excitons areformed to bring the guest material 132 into an excited state (a singletexcited state or a triplet excited state).

In the case where the formed excited state of the guest material 132 isa singlet excited state, light emission is obtained from the singletexcited state of the guest material 132. To obtain a high emissionefficiency in this case, the fluorescence quantum yield of the guestmaterial 132 is preferably high.

In the case where the guest material 132 is brought into a tripletexcited state, the triplet excited state of the guest material 132 isthermally deactivated and does not contribute to light emission becausethe guest material 132 is a fluorescent material. However, if the T1level of the host material 131 is lower than the T1 level of the guestmaterial 132, the triplet excitation energy of the guest material 132can be transferred from the T1 level of the guest material 132 to the T1level of the host material 131, which is present in greater quantitythan the guest material 132. In that case, the triplet excitation energycan be converted into singlet excitation energy by (β) TTA processdescribed later. Hence, to increase the probability of occurrence ofTTA, the T1 level of the host material 131 should be lower than the T1level of the guest material 132.

In the case where the T1 level of the host material 131 is higher thanthe T1 level of the guest material 132, the probability of carrierrecombination in the guest material 132 can be reduced when the weightpercentage of the guest material 132 is lower than that of the hostmaterial 131. In addition, the probability of energy transfer from theT1 level of the host material 131 to the T1 level of the guest material132 can be reduced. Specifically, the weight ratio of the guest material132 to the host material 131 is preferably greater than 0 and less thanor equal to 0.05.

<<(β) TTA Process>>

Described next is the case where a singlet exciton is formed fromtriplet excitons formed in the carrier recombination process in thelight-emitting layer 130.

Here, the case where the T1 level of the host material 131 is lower thanthe T1 level of the guest material 132 is described. The correlation ofenergy levels in this case is schematically shown in FIG. 1C. What termsand numerals in FIG. 1C represent are listed below. Note that the T1level of the host material 131 may be higher than the T1 level of theguest material 132.

Host (131): the host material 131

Guest (132): the guest material 132 (fluorescent material)

S_(FH): the S1 level of the host material 131

T_(FH): the T1 level of the host material 131

S_(FG): the S1 level of the guest material 132 (fluorescent material)

T_(FG): the T1 level of the guest material 132 (fluorescent material)

Carriers recombine in the host material 131 and excitons are generatedto bring the host material 131 into an excited state. In the case wherethe excitons generated at this time are triplet excitons, two of thetriplet excitons approach each other, and one of them might be convertedinto a singlet exciton having the energy of the S1 level (S_(FH)) of thehost material 131 (see TTA in FIG. 1C). This reaction is represented byGeneral Formula (G1) or (G2), where the number of triplet excitonsdecreases while singlet excitons are generated.

³H*+³H*→¹(HH)*→¹H**+H→¹H*+H  (G1)

³H*+³H*→³(HH)*→³H**+H→³H*+H  (G2)

In the reaction in General Formula (G1), a pair of excitons (¹(HH)*)with a total spin quantum number of 0 are formed from two tripletexcitons (³H*) with a total spin quantum number of 0 in the hostmaterial 131, and a singlet exciton (¹H*) is generated through anelectronically or oscillatorily excited high-order singlet exciton(¹H**). In the reaction in General Formula (G2), a pair of excitons(³(HH)*) with a total spin quantum number of 1 are formed from twotriplet excitons (³H*) with a total spin quantum number of 1 (atomicunit) in the host material 131, and a triplet exciton (³H*) is generatedthrough an electronically or oscillatorily excited high-order tripletexciton (³H**). Note that in General Formulae (G1) and (G2), Hrepresents the ground state of the host material 131.

In General Formulae (G1) and (G2), there are three times as many pairsof triplet excitons with a total spin quantum number of 1 (atomic unit)as pairs of triplet excitons with a total spin quantum number of 0. Inother words, when an exciton is formed from two triplet excitons, thesinglet-triplet exciton formation ratio is 1:3 according to thestatistically obtained probability. In the case where the density of thetriplet excitons in the light-emitting layer 130 is sufficiently high(e.g., 1×10¹² cm⁻³ or more), only the reaction of two triplet excitonsapproaching each other can be considered whereas quenching of a singletriplet exciton is ignored.

Thus, by one reaction of General Formula (G1) and three reactions ofGeneral Formula (G2), one singlet exciton (¹H*) and three high-ordertriplet excitons (³H**) which are electronically or oscillatorilyexcited are formed from eight triplet excitons (³H*).

8³H*→¹H*+3³H**+4H→¹H*+3³H*+4H  (G3)

The electronically or oscillatorily excited high-order triplet excitons(³H**), which are generated in General Formula (G3), become tripletexcitons (³H*) by rapid relaxation and then repeat the reaction inGeneral Formula (G3) again with other triplet excitons. Hence, inGeneral Formula (G3), if all the triplet excitons (³H*) are convertedinto singlet excitons (¹H*), one singlet exciton (¹H*) is generated fromfive triplet excitons (³H*) (General Formula (G4)).

5³H→¹H*+4H  (G4)

The ratio of singlet excitons (¹H*) to triplet excitons (³H*) which aredirectly formed by recombination of carriers injected from a pair ofelectrodes is statistically as follows: ¹H*: ³H*=1:3. That is, theprobability of singlet excitons being directly formed by recombinationof carriers injected from a pair of electrodes is 25%.

When the singlet excitons directly formed by recombination of carriersinjected from a pair of electrodes and the singlet excitons formed byTTA are put together, eight singlet excitons can be formed from twentyexcitons (the sum of singlet excitons and triplet excitons) directlyformed by recombination of carriers injected from a pair of electrodes(General Formula (G5)). That is, TTA can increase the probability ofsinglet exciton formation from 25%, which is the conventional value, toat most 40% (=8/20).

5¹H*+15³H*→5¹H*+(3¹H*+12H)  (G5)

In the singlet excited state of the host material 131, which is formedby the singlet excitons formed through the above process, energy istransferred from the S1 level (S_(FH)) of the host material 131 to theS1 level (S_(FG)) of the guest material 132, which is lower than S_(FH)(see Route A in FIG. 1C). Then, the guest material 132 brought into asinglet excited state emits fluorescence.

In the case where carriers recombine in the guest material 132 and anexcited state formed by the formed excitons is a triplet excited state,triplet excitation energy of T_(FG) is not deactivated and transferredto T_(FH) (see Route B in FIG. 1C) to contribute to TTA when the T1level (T_(FH)) of the host material 131 is lower than the T1 level(T_(FG)) of the guest material 132.

In the case where the T1 level (T_(FG)) of the guest material 132 islower than the T1 level (T_(FH)) of the host material 131, the weightpercentage of the guest material 132 is preferably lower than that ofthe host material 131. Specifically, the weight ratio of the guestmaterial 132 to the host material 131 is preferably greater than 0 andless than or equal to 0.05, which reduces the probability of carrierrecombination in the guest material 132. In addition, the probability ofenergy transfer from the T1 level (T_(FH)) of the host material 131 tothe T1 level (T_(FG)) of the guest material 132 can be reduced.

As described above, triplet excitons formed in the light-emitting layer130 can be converted into singlet excitons by TTA, so that lightemission from the guest material 132 can be efficiently obtained.

<Probability of TTA Occurrence>

As described above, the probability of formation of singlet excitons andthe emission efficiency of a light-emitting element can be increased byTTA; thus, an increase in the probability of occurrence of TTA (alsoreferred to as TTA efficiency) is important to achieve a high emissionefficiency. That is, a delayed fluorescence component due to TTA shouldaccount for a high proportion of light emission from the light-emittingelement.

As described above, owing to the TTA process, the probability offormation of singlet excitons can be increased to at most 40% including25% occupied by the singlet excitons that are directly formed byrecombination of carriers injected from a pair of electrodes. Thus, theproportion of a delayed fluorescence component due to TTA can beincreased to at most 37.5% ((40%−25%)/40%) of light emission from thelight-emitting element.

<Improved Emission Efficiency with Increase in Delayed FluorescenceComponent in Light Emission>

For example, in a light-emitting element that emits blue light andincudes an anthracene compound generally used as a host material, adelayed fluorescence component due to TTA accounts for approximately 10%of light emission. Note that in this specification, the delayedfluorescence refers to light that is obtained after the steady injectionof carriers to a light-emitting layer is stopped, and that iscontinuously emitted for 1×10⁻⁶ sec or longer with an intensity ratio of0.01 or more with respect to the emission intensity with carrierssteadily injected.

In order to improve the emission efficiency of a light-emitting elementthat emits blue light, the proportion of a delayed fluorescencecomponent due to TTA in light emission needs to be further increased.

As described above, in the TTA process, a singlet exciton is formed fromtriplet excitons formed in the carrier recombination process in thelight-emitting layer 130. However, if the triplet excitons formed in thecarrier recombination process are quenched in another process, they donot contribute to the formation of the singlet exciton, causing adecrease in the delayed fluorescence component due to TTA in the lightemission from the light-emitting element.

The formed triplet excitons might be quenched by a variety of factors,one of which is the action of carrier electrons in the light-emittinglayer 130. The triplet excitons formed in the light-emitting layer 130are quenched in some cases when interacting with carrier electrons.

Thus, in the light-emitting element of one embodiment of the presentinvention, the density of carrier electrons in the light-emitting layer130 is adjusted to reduce the quenching of triplet excitons. The carrierelectrons in the light-emitting layer 130 are mainly supplied from theelectron-transport layer 118; accordingly, the transfer of carrierelectrons from the electron-transport layer 118 to the light-emittinglayer 130 only needs to be adjusted. This can be achieved by making anenergy barrier between the LUMO level of a material used for theelectron-transport layer 118 and the LUMO level of the host material 131contained in the light-emitting layer 130.

In the light-emitting element of one embodiment of the presentinvention, the LUMO level of the material used for theelectron-transport layer 118 is made lower than the LUMO level of thehost material 131 contained in the light-emitting layer 130, so that anenergy barrier against the transfer of carrier electrons is formed. Whenthe transfer of carrier electrons to the light-emitting layer 130 ishindered, the carrier recombination region in the light-emitting layer130 spreads to the electron-transport layer 118 side, and both thetriplet excitons and the carrier electrons have a lower density in therecombination region, resulting in a decrease in the probability ofquenching of the triplet excitons. It is needless to say that adecreased density of triplet excitons might reduce the probability ofoccurrence of TTA itself. However, the present inventors have found thatthe effect of preventing the quenching of triplet excitons due todecreased electron density more than compensates for the adverse effectof decreased density of triplet excitons, and TTA is more likely tooccur in the above structure.

FIG. 2 shows the energy diagram, where the LUMO level of the materialused for the electron-transport layer 118 is higher or lower than theLUMO level of the host material 131 contained in the light-emittinglayer 130. It is found from FIG. 2 that a recombination region spreadsto the electron-transport layer (ETL) 118 side when an energy barrier isformed between the electron-transport layer (ETL) 118 and thelight-emitting layer (EmL) 130; then, the density of both tripletexcitons and electrons decreases, reducing the probability of quenchingof triplet excitons. A reduced probability of quenching increases thenumber of singlet excitons formed from triplet excitons in the TTAprocess, thereby increasing the delayed fluorescence component due toTTA in the light emission from the light-emitting element. As a result,the emission efficiency of the light-emitting element of one embodimentof the present invention can be improved.

In one embodiment of the present invention, the proportion of thedelayed fluorescence component due to TTA can be, for example, higherthan or equal to 10% of light emission from the light-emitting element.Furthermore, the proportion of the delayed fluorescence component due toTTA can be higher than or equal to 15% of light emission from thelight-emitting element when a material having a relatively deep LUMOlevel is used for the electron-transport layer 118. To achieve such aneffect, an appropriate energy barrier, preferably greater than or equalto 0.05 eV, is made between the LUMO level of the material used for theelectron-transport layer 118 and the LUMO level of the host material 131contained in the light-emitting layer 130.

Here, in the case where the material used for the electron-transportlayer 118 has an extremely deep LUMO level, carrier electrons areunlikely to transfer from the electron-transport layer 118 to thelight-emitting layer 130 to affect the carrier balance in thelight-emitting layer 130, which might reduce the emission efficiency ofthe light-emitting element. In contrast, the aforementioned energybarrier should be high enough to appropriately suppress the transfer ofcarrier electrons from the electron-transport layer 118 to thelight-emitting layer 130. Hence, the difference between the LUMO levelof the material used for the electron-transport layer 118 and the LUMOlevel of the host material 131 contained in the light-emitting layer 130is preferably greater than or equal to 0.05 eV and less than or equal to0.3 eV.

As described above, the light-emitting element of one embodiment of thepresent invention appropriately suppresses the transfer of carrierelectrons from the electron-transport layer 118 to the light-emittinglayer 130. Thus, when carrier electrons in the light-emitting layer 130are trapped by the guest material 132, which is present in less quantitythan the host material 131, electrons are less likely to transfer alsoin the light-emitting layer 130, causing an unnecessary increase indriving voltage. In view of this, the LUMO level of the guest materialis preferably higher than the LUMO level of the host material.

Note that a factor of delayed fluorescence in a light-emitting element,which is other than TTA, may be thermally activated delayed fluorescencedue to reverse intersystem crossing from the triplet excited state tothe singlet excited state. To efficiently cause reverse intersystemcrossing, an energy difference between the S1 level and the T1 level ispreferably less than or equal to 0.2 eV. In other words, an energydifference greater than 0.2 eV between the S1 level and the T1 levelhardly causes reverse intersystem crossing. Therefore, to efficientlycause TTA, an energy difference between the lowest singlet excitationenergy level and the lowest triplet excitation energy level of acompound in which TTA occurs is preferably greater than 0.2 eV, furtherpreferably greater than or equal to 0.5 eV.

The lowest singlet excitation energy level of an organic compound can beobserved from an absorption spectrum at a transition from the groundstate to the lowest singlet excited state in the organic compound.Alternatively, the lowest singlet excitation energy level may beestimated from a peak wavelength of a fluorescence spectrum of theorganic compound. Furthermore, the lowest triplet excitation energylevel can be observed from an absorption spectrum at a transition fromthe ground state to the lowest triplet excited state in the organiccompound, but is difficult to observe in some cases because thistransition is a forbidden transition. In such cases, the lowest tripletexcitation energy level may be estimated from a peak wavelength of aphosphorescence spectrum of the organic compound. Thus, a difference inequivalent energy value between the peak wavelengths of the fluorescenceand phosphorescence spectra of the organic compound is preferablygreater than 0.2 eV, further preferably greater than or equal to 0.5 eV.

<Hole-Transport Layer and Improvement in Emission Efficiency>

The relationship between the material contained in theelectron-transport layer 118 and the emission efficiency has beendescribed above. Next, the relationship between the material containedin the hold-transport layer 112 and the emission efficiency will bedescribed.

The material contained in the hole-transport layer 112 preferably has ahigher LUMO level than the host material 131. In the case where thematerial contained in the hole-transport layer 112 has the same LUMOlevel as the host material 131, carrier electrons reaching thelight-emitting layer 130 do not remain in the light-emitting layer 130and moves to the hole-transport layer 112. Then, carriers recombine alsoin the hole-transport layer 112, which reduces the efficiency ofrecombination of carriers in the light-emitting layer 130. This causes adecreased emission efficiency unless the energy of excitons generated inthe hole-transport layer 112 can be transferred to the light-emittingmaterial in the light-emitting layer 130.

Hence, the material contained in the hole-transport layer 112 preferablyhas a higher LUMO level than the host material 131. Note that the LUMOlevel of the material contained in the hole-transport layer 112 ispreferably higher than the LUMO level of the host material 131 bygreater than or equal to 0.3 eV, in which case the transfer of carrierelectrons from the light-emitting layer 130 to the hole-transport layer112 can be suppressed effectively.

<Suppression of Transfer of Triplet Excitation Energy>

Triplet excitation energy generated in the light-emitting layer 130remains in the light-emitting layer 130 so as not to leave thelight-emitting layer 130 in the following manner.

When the triplet excitation energy generated in the light-emitting layer130 moves outside, the probability of occurrence of TTA in thelight-emitting layer 130 decreases. In other words, the suppression ofthe transfer of the triplet excitation energy results in maintaining ahigh probability of occurrence of TTA in the light-emitting layer and ahigh emission efficiency of the light-emitting element.

First, to suppress the transfer of triplet excitation energy from thelight-emitting layer 130 to the hole-transport layer 112, the T1 levelof the material contained in the hole-transport layer 112 is preferablymade higher than the T1 level of the host material 131 contained in thelight-emitting layer 130, more preferably, by greater than or equal to0.2 eV.

Similarly, to suppress the transfer of triplet excitation energy fromthe light-emitting layer 130 to the electron-transport layer 118, the T1level of the material contained in the electron-transport layer 118 ispreferably made higher than the T1 level of the host material 131contained in the light-emitting layer 130, more preferably, by greaterthan or equal to 0.2 eV.

When triplet excitation energy is prevented from moving and remains inthe light-emitting layer 130, the triplet excitation energy is likely tobe lost only due to TTA, so that the probability of occurrence of TTA inthe light-emitting layer 130 and the emission efficiency of thelight-emitting element can be maintained high.

<Measurement of Delayed Fluorescence Component>

Described is an example of a method for measuring the delayedfluorescence component in light emission from a light-emitting layer.

When carriers are steadily injected to the light-emitting layer, lightemission from the light-emitting layer has an intensity including adelayed fluorescence component and other components. The emissionintensity relating to the delayed fluorescence reaches a maximum whencarriers are injected to the light-emitting layer for a sufficientperiod of time. Thus, the proportion of a delayed fluorescence componentin light emission refers to a value in a state where carriers aresteadily injected to the light-emitting layer.

The proportion of a delayed fluorescence component in light emission maybe measured by stopping the injection of carriers to the light-emittinglayer and measuring the attenuated light. After carrier injection isstopped, the fluorescence usually quenches in several nanoseconds whilethe delayed fluorescence quenches in several microseconds. Accordingly,the delayed fluorescence can be measured by observing the component thatquenches in several microseconds.

The attenuation of light is observed with a streak camera for severalmicroseconds after the injection of carriers to the light-emitting layeris stopped, whereby an exponential attenuation curve can be obtained.The light emission includes a delayed fluorescence component and othercomponents just after the carrier injection to the light-emitting layeris stopped; after several nanoseconds or more, only the delayedfluorescence component remains in effect. Hence, by fitting theattenuation curve with an exponential function, an attenuation curveformula with the time as a parameter can be obtained.

The time 0 s is substituted to the attenuation curve formula to estimatethe intensity value of the delayed fluorescence component at the time ofstopping the carrier injection. Carriers are steadily injected at themoment of stopping the carrier injection to the light-emitting layer,i.e., the estimated intensity of the delayed fluorescence component isequal to the intensity of the delayed fluorescence component at the timewhen carriers are steadily injected. The proportion of the delayedfluorescence component in light emission can be calculated from theobtained intensity of the delayed fluorescence component and theemission intensity of the light-emitting layer to which carriers aresteadily injected.

Note that the delayed fluorescence component in light emission from thelight-emitting layer might include not only delayed fluorescence derivedfrom the TTA process with intermolecular interaction but also thermallyactivated delayed fluorescence (TADF) derived from the energy transferof a molecule from a triplet excitation energy level to a singletexcitation energy level. The TADF is generated under the followingconditions, which enable the reverse energy transfer from the tripletexcitation energy level to the singlet excitation energy level. Both ofthe energy levels need to be close to each other, specifically, theenergy gap therebetween should be less than or equal to 0.2 eV to causethe TADF. However, only some of the molecules used for thelight-emitting layer satisfy the conditions. Thus, unless a moleculewith a small energy gap is used in a light-emitting layer, the TADF doesnot need to be considered and the delayed fluorescence component inlight emission from the light-emitting layer can be substantiallyderived from the TTA process.

For specific measurements, Examples can be referred to.

<Molecular Orientation and Outcoupling Efficiency>

In organic EL, carriers are supplied to a light-emitting layer andrecombine therein, so that light is emitted from a guest materialcontained in the light-emitting layer. In some cases, the light emissionis anisotropic, i.e., has angle-dependent intensity. The light emissionis perpendicular to the transition dipole moment of the guest material;accordingly, the transition dipole moment orientation influences theangular dependence of the light emission. Since the transition dipolemoment orientation of an organic molecule is affected by the molecularorientation of the organic molecule, light emission from the guestmaterial sometimes has anisotropy due to the molecular orientation ofthe guest material.

The light-emitting layer includes a plurality of molecules and the guestmaterial is dispersed in the host material. In some fabricationconditions of the light-emitting layer, the guest molecules are notrandomly oriented in the host material but are oriented in a direction,that is, the guest molecules may have uneven molecular orientation. Ifthe guest material in the light-emitting layer has orientation thatallows light to be easily extracted from a light-emitting element, theoutcoupling efficiency of the light-emitting element is improved.Specifically, the guest molecules are preferably oriented so that theirtransition dipole moment is horizontal to a substrate surface.

In the estimation of the molecular orientation in an actuallight-emitting element, it is not easy to directly observe thetransition dipole moment orientation of a molecule, or morespecifically, a guest material in a light-emitting layer. Therefore, toestimate the molecular orientation of a light-emitting material in thelight-emitting layer, the present inventors have thought of a method inwhich light emitted from the light-emitting layer is linearly polarizedto extract a p-polarized component, the angular dependence of theintegrated intensity of the obtained p-polarized emission spectrum fromthe visible to near-infrared region (from 440 nm to 956 nm) is measuredand analyzed by calculation (simulation). The estimation method ofmolecular orientation will be described below.

When guest molecules are randomly oriented in host molecules, thefollowing state is obtained. The total transition dipole moment of allthe molecules has the same component in the x direction, the ydirection, and the z direction which are orthogonal to one another. Forexample, in the case where a layer is present on a plane along the xdirection and the y direction and molecules in the layer areisotropically oriented, a transition dipole moment component parallel tothe layer, which has two dimensions, is two-thirds (67%) of the entirecomponent, and a component perpendicular to the layer is one-third (33%)of the entire component.

The measurement will be described next. In the measurement of theintensity of light from a light-emitting layer, the light enters aGlan-Taylor polarizer and passes therethrough before entering adetector. Thus, only a polarization component in a specific directioncan be detected by the detector.

Here, three types of components of the transition dipole moment of lightare determined as shown in FIGS. 3A to 3C: A) a transition dipole momentcomponent 181 which is parallel to the light-emitting layer 130 and in adirection parallel to an observation direction 180 of the detector; B) atransition dipole moment component 182 which is parallel to thelight-emitting layer 130 and in a direction perpendicular to theobservation direction 180 of the detector; and C) a transition dipolemoment component 183 which is in a direction perpendicular to thelight-emitting layer 130. The component B cannot pass through theGlan-Taylor polarizer between the detector and the light-emitting layer130, and therefore is not detected by the detector. In other words,p-polarized emission including the components A and C is observed inthis measurement.

Next, to measure the angular dependence of light emission, thelight-emitting layer 130 is gradually inclined from the initial positionwhere the light-emitting layer 130 is in a direction perpendicular to adetector 185. FIG. 4A shows the initial state and FIG. 4B shows thetilted light-emitting layer 130 (tilt angle θ). In the initial state(tilt angle=0°), the detector 185, which is in front of thelight-emitting layer 130, does not measure light derived from theaforementioned component C, but measures the component A. As the tiltangle of the light-emitting layer 130 increases, not only the componentA but also the component C is gradually measured by the detector 185depending on the tilt angle. In this manner, the angular dependence oflight emission can be measured.

In the light extracted from the element, the component perpendicular tothe light-emitting layer 130 has much lower intensity than the componentparallel to the light-emitting layer 130; in that case, the component Cis difficult to measure. Thus, the thickness of each layer in thelight-emitting element is adjusted in advance, and the emissionintensity of the component parallel to the light-emitting layer 130 isreduced by utilizing optical interference. Light extracted straight fromthe light-emitting element includes a component extracted directly fromthe light-emitting layer 130 and a component that is extracted afterlight generated in the light-emitting layer 130 enters an electrode andreflects off it. The phases of the two components are reversed andcanceled out by adjusting the thickness of each layer in thelight-emitting element. This can weaken the component A, facilitatingthe observation of the component C.

The angular dependence of light emission from the light-emitting layercan be measured in the above manner. The measured results are plotted toobtain a graph, where the horizontal axis represents the tilt angle ofthe light-emitting layer 130 from the initial state and the verticalaxis represents the normalized integrated intensity of emissionspectrum. The shape of the graph changes with the ratio of the componentA to the component C in the light emission. The shape of each graph witha different ratio of the component A to the component C in the lightemission can be obtained by calculation (simulation). In other words,the ratio of the component A to the component C in the light emissioncan be obtained by fitting the graph as the result of the calculation tothe graph as the result of the measurement. Each molecule (guestmaterial in this case) has a unique transition dipole momentorientation; hence, the information on the orientation of the guestmaterial can be obtained from the ratio of the component A to thecomponent C.

The component A exceeding 67% means a large amount of transition dipolemoment component parallel to the light-emitting layer; briefly, 100% ofthe component A means a completely horizontal orientation. Because lightis emitted in a direction perpendicular to the transition dipole moment,the outcoupling efficiency increases as the transition dipole momentbecomes more parallel to the light-emitting layer. That is, the emissionefficiency of the element increases as the component A approaches 100%.

Note that when light emitted from the light-emitting element of oneembodiment of the present invention is observed in the abovemeasurement, it is found that the guest material is oriented notrandomly but in a specific direction, and the transition dipole momentsignificantly deviates from the direction perpendicular to thelight-emitting layer. The intensity of light emission in the directionperpendicular to the light-emitting layer increases as the transitiondipole moment becomes deviating from the direction perpendicular to thelight-emitting layer. This indicates that the orientation of the guestmaterial contributes to the high emission efficiency of thelight-emitting element of one embodiment of the present invention.

Note that for details of the measurement and calculation, thedescription of Examples can also be referred to.

<Materials>

Next, components of the light-emitting element of one embodiment of thepresent invention will be described in detail.

<<Light-Emitting Layer>>

In the light-emitting layer 130, the weight percentage of the hostmaterial 131 is higher than that of at least the guest material 132, andthe guest material 132 (fluorescent material) is dispersed in the hostmaterial 131. The host material 131 in the light-emitting layer 130 ispreferably an organic compound in which delayed fluorescence componentsdue to triplet-triplet annihilation (TTA) account for a high proportionof emitted light; specifically, an organic compound in which delayedfluorescence components due to TTA account for 20% or more. Note that inthe light-emitting layer 130, the host material 131 may be composed ofone kind of compound or a plurality of compounds.

In the light-emitting layer 130, the guest material 132 is preferably,but not particularly limited to, an anthracene derivative, a tetracenederivative, a chrysene derivative, a phenanthrene derivative, a pyrenederivative, a perylene derivative, a stilbene derivative, an acridonederivative, a coumarin derivative, a phenoxazine derivative, aphenothiazine derivative, or the like, and for example, any of thefollowing materials can be used.

The examples include5,6-bis[4-(10-phenyl-9-anthryl)phenyl]-2,2′-bipyridine (abbreviation:PAP2BPy), 5,6-bis[4′-(10-phenyl-9-anthryl)biphenyl-4-yl]-2,2′-bipyridine(abbreviation: PAPP2BPy),N,N′-diphenyl-N,N′-bis[4-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6FLPAPrn),N,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn),N,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N′-diphenylstilbene-4,4′-diamine(abbreviation: YGA2S),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),4-(9H-carbazol-9-yl)-4′-(9,10-diphenyl-2-anthryl)triphenylamine(abbreviation: 2YGAPPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA), perylene, 2,5,8,11-tetra(tert-butyl)perylene(abbreviation: TBP),4-(10-phenyl-9-anthryl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBAPA),N,N″-(2-tert-butylanthracene-9,10-diyldi-4,1-phenylene)bis[N,N′,N′-triphenyl-1,4-phenylenediamine](abbreviation: DPABPA),N,9-diphenyl-N-[4-(9,10-diphenyl-2-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: 2PCAPPA),N-[4-(9,10-diphenyl-2-anthryl)phenyl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPPA),N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), coumarin 30,N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,9-diphenyl-9H-carbazol-3-amine(abbreviation: 2PCABPhA),N-(9,10-diphenyl-2-anthryl)-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPAPA),N-[9,10-bis(1,1′-biphenyl-2-yl)-2-anthryl]-N,N′,N′-triphenyl-1,4-phenylenediamine(abbreviation: 2DPABPhA),9,10-bis(1,1′-biphenyl-2-yl)-N-[4-(9H-carbazol-9-yl)phenyl]-N-phenylanthracen-2-amine(abbreviation: 2YGABPhA), N,N,9-triphenylanthracen-9-amine(abbreviation: DPhAPhA), coumarin 6, coumarin 545T,N,N′-diphenylquinacridone (abbreviation: DPQd), rubrene,5,12-bis(1,1′-biphenyl-4-yl)-6,11-diphenyltetracene (abbreviation: BPT),2-(2-{2-[4-(dimethylamino)phenyl]ethenyl}-6-methyl-4H-pyran-4-ylidene)propanedinitrile(abbreviation: DCM1),2-{2-methyl-6-[2-(2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCM2),N,N,N′,N′-tetrakis(4-methylphenyl)tetracene-5,11-diamine (abbreviation:p-mPhTD),7,14-diphenyl-N,N,N′,N′-tetrakis(4-methylphenyl)acenaphtho[1,2-a]fluoranthene-3,10-diamine(abbreviation: p-mPhAFD),2-{2-isopropyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTI),2-{2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: DCJTB), 2-(2,6-bis{2-[4-(dimethylamino)phenyl]ethenyl}-4H-pyran-4-ylidene)propanedinitrile(abbreviation: BisDCM),2-{2,6-bis[2-(8-methoxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-benzo[ij]quinolizin-9-yl)ethenyl]-4H-pyran-4-ylidene}propanedinitrile(abbreviation: BisDCJTM), and5,10,15,20-tetraphenylbisbenzo[5,6]indeno[1,2,3-cd: 1′,2′,3′-lm]perylene.

Note that the light-emitting layer 130 may include a material other thanthe host material 131 and the guest material 132.

Although there is no particular limitation on a material that can beused in the light-emitting layer 130, any of the following materials canbe used, for example: metal complexes such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum(III) (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), andbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ);heterocyclic compounds such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ),2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), and9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11); and aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB). In addition, condensed polycyclic aromaticcompounds such as anthracene derivatives, phenanthrene derivatives,pyrene derivatives, chrysene derivatives, and dibenzo[g,p]chrysenederivatives can be used. Specific examples thereof include9,10-diphenylanthracene (abbreviation: DPAnth),N,N-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: CzA1PA), 4-(10-phenyl-9-anthryl)triphenylamine(abbreviation: DPhPA),4-(9H-carbazol-9-yl)-4′-(10-phenyl-9-anthryl)triphenylamine(abbreviation: YGAPA),N,9-diphenyl-N-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol-3-amine(abbreviation: PCAPA),N,9-diphenyl-N-{4-[4-(10-phenyl-9-anthryl)phenyl]phenyl}-9H-carbazol-3-amine(abbreviation: PCAPBA),N,9-diphenyl-N-(9,10-diphenyl-2-anthryl)-9H-carbazol-3-amine(abbreviation: 2PCAPA), 6,12-dimethoxy-5,11-diphenylchrysene,N,N,N′,N′,N″,N″,N′″,N′″-octaphenyldibenzo[g,p]chrysene-2,7,10,15-tetraamine(abbreviation: DBC1), 9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazol(abbreviation: CzPA),3,6-diphenyl-9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole(abbreviation: DPCzPA), 9,10-bis(3,5-diphenylphenyl)anthracene(abbreviation: DPPA), 9,10-di(2-naphthyl)anthracene (abbreviation: DNA),2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,9′-bianthryl (abbreviation: BANT),9,9′-(stilbene-3,3′-diyl)diphenanthrene (abbreviation: DPNS),9,9′-(stilbene-4,4′-diyl)diphenanthrene (abbreviation: DPNS2), and1,3,5-tri(1-pyrenyl)benzene (abbreviation: TPB3). One or more substanceshaving a wider energy gap than the guest material 132 is preferablyselected from these substances and known substances.

Note that the light-emitting layer 130 can have a structure in which twoor more layers are stacked. For example, in the case where thelight-emitting layer 130 is formed by stacking a first light-emittinglayer and a second light-emitting layer in this order from thehole-transport layer side, a substance having a hole-transport propertyis used as the host material of the first light-emitting layer and asubstance having an electron-transport property is used as the hostmaterial of the second light-emitting layer. Alternatively, thelight-emitting layer 130 may include a first region containing a hostmaterial and a guest material and a second region containing a hostmaterial.

Next, details of other components of the light-emitting element 150 inFIG. 1A will be described below.

<<Pair of Electrodes>>

The electrode 101 and the electrode 102 have functions of injectingholes and electrons into the light-emitting layer 130. The electrodes101 and 102 can be formed using a metal, an alloy, or a conductivecompound, or a mixture or a stack thereof, for example. A typicalexample of the metal is aluminum, besides, a transition metal such assilver, tungsten, chromium, molybdenum, copper, or titanium, an alkalimetal such as lithium or cesium, or a Group 2 metal such as calcium ormagnesium can be used. As the transition metal, a rare earth metal suchas ytterbium (Yb) may be used. An alloy containing any of the abovemetals can be used as the alloy, and MgAg and AlLi can be given asexamples. As the conductive compound, a metal oxide such as indiumoxide-tin oxide (indium tin oxide) can be given. It is also possible touse an inorganic carbon-based material such as graphene as theconductive compound. As described above, the electrode 101 and/or theelectrode 102 may be formed by stacking two or more of these materials.

Light emitted from the light-emitting layer 130 is extracted through theelectrode 101 and/or the electrode 102. Therefore, at least one of theelectrodes 101 and 102 transmits visible light. In the case where theelectrode through which light is extracted is formed using a materialwith low light transmittance, such as metal or alloy, the electrode 101and/or the electrode 102 is formed to a thickness that is thin enough totransmit visible light (e.g., a thickness of 1 nm to 10 nm).

<<Hole-Injection Layer>>

The hole-injection layer 111 has a function of reducing a barrier forhole injection from one of the pair of electrodes (the electrode 101 orthe electrode 102) to promote hole injection and is formed using, forexample, a transition metal oxide, a phthalocyanine derivative, or anaromatic amine. As the transition metal oxide, molybdenum oxide,vanadium oxide, ruthenium oxide, tungsten oxide, manganese oxide, or thelike can be given. As the phthalocyanine derivative, phthalocyanine,metal phthalocyanine, or the like can be given. As the aromatic amine, abenzidine derivative, a phenylenediamine derivative, or the like can begiven. It is also possible to use a high molecular compound such aspolythiophene or polyaniline; a typical example thereof ispoly(ethylenedioxythiophene)/poly(styrenesulfonic acid), which isself-doped polythiophene.

As the hole-injection layer 111, a layer containing a composite materialof a hole-transport material and a material having a property ofaccepting electrons from the hole-transport material can also be used.Alternatively, a stack of a layer containing a material having anelectron accepting property and a layer containing a hole-transportmaterial may also be used. In a steady state or in the presence of anelectric field, electric charges can be transferred between thesematerials. As examples of the material having an electron-acceptingproperty, organic acceptors such as a quinodimethane derivative, achloranil derivative, and a hexaazatriphenylene derivative can be given.A specific example is a compound having an electron-withdrawing group (ahalogen group or a cyano group), such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F4-TCNQ), chloranil, or2,3,6,7,10,11-hexacyano-1,4,5,8,9,12-hexaazatriphenylene (abbreviation:HAT-CN). Alternatively, a transition metal oxide such as an oxide of ametal from Group 4 to Group 8 can also be used. Specifically, vanadiumoxide, niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, rhenium oxide, or the like can be used.In particular, molybdenum oxide is preferable because it is stable inthe air, has a low hygroscopic property, and is easily handled.

A material having a property of transporting more holes than electronscan be used as the hole-transport material, and a material having a holemobility of 1×10⁻⁶ cm²/Vs or higher is preferable. Specifically, anaromatic amine, a carbazole derivative, an aromatic hydrocarbon, astilbene derivative, or the like can be used. Furthermore, thehole-transport material may be a high molecular compound.

Examples of the aromatic amine compound, which has a high hole-transportproperty, include N,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine(abbreviation: DTDPPA),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4diamine (abbreviation: DNTPD), and1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B).

Specific examples of the carbazole derivative are3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation:PCPPn).

Other examples of the carbazole derivative include4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation: CzPA), and1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene.

Examples of the aromatic hydrocarbon are2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl) anthracene (abbreviation:t-BuDBA), 9,10-di(2-naphthyl) anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl) anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, and 2,5,8,11-tetra(tert-butyl)perylene.Other examples are pentacene and coronene. The aromatic hydrocarbonhaving a hole mobility of 1×10⁻⁶ cm²/Vs or more and having 14 to 42carbon atoms is particularly preferable.

The aromatic hydrocarbon may have a vinyl skeleton. Examples of thearomatic hydrocarbon having a vinyl group are4,4′-bis(2,2-diphenylvinyl)biphenyl (abbreviation: DPVBi), and9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene (abbreviation: DPVPA).

Other examples are high molecular compounds such aspoly(N-vinylcarbazole) (abbreviation: PVK), poly(4-vinyltriphenylamine)(abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), andpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:poly-TPD).

«Hole-Transport Layer>>

The hole-transport layer 112 is a layer containing a hole-transportmaterial and can be formed using any of the materials given as examplesof the material of the hole-injection layer 111. In order that thehole-transport layer 112 has a function of transporting holes injectedinto the hole-injection layer 111 to the light-emitting layer 130, thehighest occupied molecular orbital (HOMO) level of the hole-transportlayer 112 is preferably equal or close to the HOMO level of thehole-injection layer 111.

In addition to the materials given as the material for thehole-injection layer 111, any of the following substances having a highhole-transport property can be used as the hole-transport material:aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), and4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP),and the like. The substances listed here are mainly substances having ahole mobility of 1×10⁻⁶ cm²/Vs or higher. Note that any substance otherthan the substances listed here may be used as long as thehole-transport property is higher than the electron-transport property.The layer including a substance having a high hole-transport property isnot limited to a single layer, and two or more layers containing theaforementioned substances may be stacked.

The hole-transport material contained in the hole-transport layer 112preferably has a higher LUMO level and a higher lowest tripletexcitation energy (T1) level than the host material 131 in thelight-emitting layer. In the case where the material contained in thehole-transport layer 112 has the same LUMO level as the host material131, carrier electrons reaching the light-emitting layer 130 do notremain in the light-emitting layer 130 and moves to the hole-transportlayer 112. Then, fewer excitons recombine in the light-emitting layer130, which reduces the emission efficiency. In the case where the lowesttriplet excitation energy (T1) level is equal to that of the hostmaterial 131, TTA does not occur from triplet excitons generated in thelight-emitting layer 130, so that the triplet energy diffuses to thehole-transport layer 112, causing a reduced emission efficiency.

For example, 3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole(abbreviation: PCPPn) is preferably used as the hole-transport materialcontained in the hole-transport layer 112. PCPPn has appropriately highLUMO level and T1 level; thus, diffusion of carrier electrons from thelight-emitting layer 130 to the hole-transport layer 112 can beappropriately suppressed, increasing the probability of occurrence ofTTA in the light-emitting layer 130 and the emission efficiency of thelight-emitting element.

<<Electron-Transport Layer>>

The electron-transport layer 118 has a function of transporting, to thelight-emitting layer 130, electrons injected from the other of the pairof electrodes (the electrode 101 or the electrode 102) through theelectron-injection layer 119. A material having a property oftransporting more electrons than holes can be used as anelectron-transport material, and a material having an electron mobilityof 1×10⁻⁶ cm²/Vs or higher is preferred. Specific examples include ametal complex having a quinoline ligand, a benzoquinoline ligand, anoxazole ligand, or a thiazole ligand; an oxadiazole derivative; atriazole derivative; a phenanthroline derivative; a pyridine derivative;a bipyridine derivative; and a pyrimidine derivative.

Specific examples include metal complexes having a quinoline orbenzoquinoline skeleton, such as tris(8-quinolinolato)aluminum(III)(abbreviation: Alq), tris(4-methyl-8-quinolinolato)aluminum(III)(abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium(II)(abbreviation: BeBq₂), and bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III) (abbreviation: BAlq). Alternatively, ametal complex having an oxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc(II) (abbreviation: Zn(BOX)₂)or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc(II) (abbreviation:Zn(BTZ)₂) can be used. Other than the metal complexes, any of thefollowing can be used:2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen), andbathocuproine (abbreviation: BCP). The substances described here aremainly substances having an electron mobility of 1×10⁻⁶ cm²/Vs orhigher. The electron-transport layer 118 is not limited to a singlelayer, and two or more layers containing the aforementioned substancesmay be stacked.

In particular, as an electron-transport material with a deep LUMO,2,2′-(pyridine-2,6-diyl)bis(4,6-diphenylpyrimidine) (abbreviation:2,6(P2Pm)2Py), 2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline(abbreviation: NBPhen),2,2′-(pyridine-2,6-diyl)bis(4-phenylbenzo[h]quinazoline) (abbreviation:2,6(P-Bqn)2Py), or the like is preferably used.

As another electron-transport material with a deep LUMO, a substanceincluding a condensed heteroaromatic ring skeleton including a diazineskeleton or a triazine skeleton in its molecular structure is preferablyused. A substance including a pyrazine skeleton or a pyrimidine skeletonin its molecular structure is also preferably used.

The LUMO level of the material used for the electron-transport layer 118is preferably made lower than the LUMO level of the host material 131contained in the light-emitting layer 130, in which case an energybarrier against the transfer of carrier electrons can be formed. Theenergy barrier hinders the transfer of carrier electrons to thelight-emitting layer 130, so that the carrier recombination region inthe light-emitting layer 130 spreads to the electron-transport layer 118side, and both the triplet excitons and the carrier electrons have alower density in the recombination region, resulting in a decrease inthe quenching of the excitons caused by the injection of carrierelectrons to the triplet excitons.

Between the electron-transport layer 118 and the light-emitting layer130, a layer that controls the transport of electron carriers may beprovided. This is a layer formed by the addition of a small amount of asubstance having a high electron-trapping property to the aforementionedmaterial having a high electron-transport property, and the layer iscapable of adjusting carrier balance by retarding the transport ofelectron carriers. Such a structure is very effective in preventing aproblem (such as a reduction in element lifetime) caused when electronspass through the light-emitting layer.

<<Electron-Injection Layer>>

The electron-injection layer 119 has a function of reducing a barrierfor electron injection from the electrode 102 to promote electroninjection and can be formed using a Group 1 metal or a Group 2 metal, oran oxide, a halide, or a carbonate of any of the metals, for example.Alternatively, a composite material containing an electron-transportmaterial (described above) and a material having a property of donatingelectrons to the electron-transport material can also be used. As thematerial having an electron-donating property, a Group 1 metal, a Group2 metal, an oxide of any of the metals, or the like can be given.

Note that the light-emitting layer, the hole-injection layer, thehole-transport layer, the electron-transport layer, and theelectron-injection layer described above can each be formed by anevaporation method (including a vacuum evaporation method), an inkjetmethod, a coating method, a gravure printing method, or the like.Besides the above-mentioned materials, an inorganic compound or a highmolecular compound (e.g., an oligomer, a dendrimer, or a polymer) may beused in the light-emitting layer, the hole-injection layer, thehole-transport layer, the electron-transport layer, and theelectron-injection layer.

<<Substrate>>

The light-emitting element 150 may be fabricated over a substrate ofglass, plastic, or the like. As the way of stacking layers over thesubstrate, layers may be sequentially stacked from the electrode 101side or sequentially stacked from the electrode 102 side.

Note that, for example, glass, quartz, plastic, or the like can be usedfor the substrate over which the light-emitting element 150 can beformed. Alternatively, a flexible substrate can be used. The flexiblesubstrate is a substrate that can be bent, such as a plastic substratemade of polycarbonate or polyarylate, for example. A film, an inorganicfilm formed by evaporation, or the like can also be used. Note thatmaterials other than these can be used as long as they can function as asupport in a manufacturing process of the light-emitting element and anoptical element or as long as they have a function of protecting thelight-emitting element and the optical element.

The light-emitting element 150 can be formed using a variety ofsubstrates, for example. The type of substrate is not limited to acertain type. As the substrate, a semiconductor substrate (e.g., asingle crystal substrate or a silicon substrate), an SOI substrate, aglass substrate, a quartz substrate, a plastic substrate, a metalsubstrate, a stainless steel substrate, a substrate including stainlesssteel foil, a tungsten substrate, a substrate including tungsten foil, aflexible substrate, an attachment film, paper including a fibrousmaterial, a base material film, or the like can be used. Examples of theglass substrate include a barium borosilicate glass substrate, analuminoborosilicate glass substrate, and a soda lime glass substrate.Examples of the flexible substrate, the attachment film, the basematerial film, and the like are substrates of plastics typified bypolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polyether sulfone (PES), and polytetrafluoroethylene (PTFE). Anotherexample is a resin such as acrylic. Other examples are polypropylene,polyester, polyvinyl fluoride, and polyvinyl chloride. Other examplesare polyamide, polyimide, aramid, epoxy, an inorganic film formed byevaporation, and paper.

Alternatively, a flexible substrate may be used as the substrate, andthe light-emitting element may be provided directly on the flexiblesubstrate. Alternatively, a separation layer may be provided between thesubstrate and the light-emitting element. The separation layer can beused when part or the whole of the light-emitting element formed overthe separation layer is completed, separated from the substrate, andtransferred to another substrate. In such a case, the light-emittingelement can be transferred to a substrate having low heat resistance ora flexible substrate as well. For the above separation layer, a stackincluding inorganic films, which are a tungsten film and a silicon oxidefilm, or a resin film of polyimide or the like formed over a substratecan be used, for example.

In other words, after the light-emitting element is formed using asubstrate, the light-emitting element may be transferred to anothersubstrate. Examples of a substrate to which the light-emitting elementis transferred include, in addition to the above-described substrates, acellophane substrate, a stone substrate, a wood substrate, a clothsubstrate (including a natural fiber (e.g., silk, cotton, or hemp), asynthetic fiber (e.g., nylon, polyurethane, or polyester), a regeneratedfiber (e.g., acetate, cupra, rayon, or regenerated polyester), a leathersubstrate, and a rubber substrate. By using such a substrate, alight-emitting element with high durability, a light-emitting elementwith high heat resistance, a lightweight light-emitting element, or athin light-emitting element can be obtained.

The light-emitting element 150 may be formed over an electrodeelectrically connected to a field-effect transistor (FET), for example,which is formed over the above-mentioned substrate, so that an activematrix display device in which the FET controls the drive of thelight-emitting element 150 can be manufactured.

One embodiment of the present invention has been described in thisembodiment. Another embodiment will also be described in the otherembodiments. Note that one embodiment of the present invention is notlimited to these. For example, one embodiment of the present inventionshows, but is not limited to, an example in which the proportion of adelayed fluorescence component due to TTA is greater than or equal to10% of light emission from an EL layer and the LUMO level of a materialcontained in an electron-transport layer is lower than the LUMO level ofa host material contained in a light-emitting layer. Depending oncircumstances or conditions, in one embodiment of the present invention,the delayed fluorescence component need not account for greater than orequal to 10% of light emission from the EL layer. Alternatively, theLUMO level of the material contained in the electron-transport layer maybe higher than the LUMO level of the host material. As another example,one embodiment of the present invention shows, but is not limited to, anexample in which the LUMO level of a material contained in anelectron-transport layer is lower than the LUMO level of a host materialcontained in a light-emitting layer by greater than or equal to 0.05 eV.Depending on circumstances or conditions, in one embodiment of thepresent invention, for example, the LUMO level of the material containedin the electron-transport layer need not be lower than the LUMO level ofthe host material contained in the light-emitting layer by greater thanor equal to 0.05 eV.

The structure described above in this embodiment can be combined withany of the structures described in the other embodiments as appropriate.

Embodiment 2

In this embodiment, structure examples of the light-emitting element ofone embodiment of the present invention, which is described inEmbodiment 1, will be described with reference to FIG. 5 to FIG. 7B.

<Structure Example 1 of Light-Emitting Element>

A structure example of the light-emitting element of one embodiment ofthe present invention will be described below with reference to FIG. 5.FIG. 5 is a cross-sectional view illustrating a light-emitting elementof one embodiment of the present invention.

A light-emitting element 250 in FIG. 5 has a bottom-emission structurein which light is extracted through a substrate 200. However, oneembodiment of the present invention is not limited to this structure andmay have a top-emission structure in which light emitted from thelight-emitting element is extracted in the direction opposite to thesubstrate 200 or a dual-emission structure in which light emitted fromthe light-emitting element is extracted in both top and bottomdirections of the substrate 200 over which the light-emitting element isformed.

The light-emitting element 250 includes the electrode 101 and theelectrode 102 over the substrate 200. Between the electrodes 101 and102, a light-emitting layer 123B, a light-emitting layer 123G, and alight-emitting layer 123R are provided. The hole-injection layer 111,the hole-transport layer 112, the electron-transport layer 118, and theelectron-injection layer 119 are also provided.

In the case where the light-emitting element has a bottom emissionstructure, the electrode 101 preferably has a function of transmittinglight and the electrode 102 preferably has a function of reflectinglight.

In the light-emitting element 250 illustrated in FIG. 5, a partitionwall 140 is provided between a region 221B, a region 221G, and a region221R, which are sandwiched between the electrode 101 and the electrode102. The partition wall 140 has an insulating property. The partitionwall 140 covers end portions of the electrode 101 and has openingsoverlapping with the electrode. With the partition wall 140, theelectrode 101 provided over the substrate 200 in the regions can bedivided into island shapes.

The light-emitting layers 123B, 123G, and 123R preferably containlight-emitting materials having functions of emitting light of differentcolors. For example, when the light-emitting layer 123B, thelight-emitting layer 123G, and the light-emitting layer 123R containlight-emitting materials having functions of emitting blue light, greenlight, and red light, respectively, the light-emitting element 250 canbe used in a full-color display device. The thicknesses of thelight-emitting layers may be the same or different.

As described in Embodiment 1, the LUMO level of a material used in theelectron-transport layer 118 is made lower than the LUMO level of a hostmaterial contained in the light-emitting layer 123B. This allows thefabrication of a light-emitting element in which a delayed fluorescencecomponent accounts for a relatively high proportion of light emissionfrom the light-emitting layer 123B.

Note that one or more of the light-emitting layers 123B, 123G, and 123Rmay include two or more stacked layers.

<Structure Example 2 of Light-Emitting Element>

Next, structure examples different from the light-emitting elementillustrated in FIG. 5 will be described below with reference to FIGS. 6Aand 6B.

FIGS. 6A and 6B are cross-sectional views of a light-emitting element ofone embodiment of the present invention. In FIGS. 6A and 6B, a portionhaving a function similar to that in FIG. 5 is represented by the samehatch pattern as in FIG. 5 and not especially denoted by a referencenumeral in some cases. In addition, common reference numerals are usedfor portions having similar functions, and a detailed description ofsuch portions is not repeated in some cases.

FIGS. 6A and 6B each illustrate a structure example of a tandemlight-emitting element in which a plurality of light-emitting layers arestacked between a pair of electrodes with a charge-generation layer 115provided between the light-emitting layers. A light-emitting element 252illustrated in FIG. 6A has a top-emission structure in which light isextracted in a direction opposite to the substrate 200, and alight-emitting element 254 illustrated in FIG. 6B has a bottom-emissionstructure in which light is extracted through the substrate 200.However, one embodiment of the present invention is not limited to thesestructures and may have a dual-emission structure in which light emittedfrom the light-emitting element is extracted in both top and bottomdirections of the substrate 200 over which the light-emitting element isformed.

The light-emitting elements 252 and 254 each include the electrode 101,the electrode 102, an electrode 103, and an electrode 104 over thesubstrate 200. A light-emitting layer 160, the charge-generation layer115, and a light-emitting layer 170 are provided between the electrode101 and the electrode 102, between the electrode 102 and the electrode103, and between the electrode 102 and the electrode 104. Thehole-injection layer 111, the hole-transport layer 112, anelectron-transport layer 113, an electron-injection layer 114, ahole-injection layer 116, a hole-transport layer 117, theelectron-transport layer 118, and the electron-injection layer 119 arefurther provided.

The electrode 101 includes a conductive layer 101 a and a conductivelayer 101 b over and in contact with the conductive layer 101 a. Theelectrode 103 includes a conductive layer 103 a and a conductive layer103 b over and in contact with the conductive layer 103 a. The electrode104 includes a conductive layer 104 a and a conductive layer 104 b overand in contact with the conductive layer 104 a.

In the light-emitting element 252 illustrated in FIG. 6A and thelight-emitting element 254 illustrated in FIG. 6B, the partition wall140 is provided between a region 222B sandwiched between the electrode101 and the electrode 102, a region 222G sandwiched between theelectrode 102 and the electrode 103, and a region 222R sandwichedbetween the electrode 102 and the electrode 104. The partition wall 140has an insulating property. The partition wall 140 covers end portionsof the electrodes 101, 103, and 104 and has openings overlapping withthe electrodes. With the partition wall 140, the electrodes providedover the substrate 200 in the regions can be divided into island shapes.

The light-emitting elements 252 and 254 each include a substrate 220provided with an optical element 224B, an optical element 224G, and anoptical element 224R in the direction in which light emitted from theregion 222B, light emitted from the region 222G, and light emitted fromthe region 222R are extracted, respectively. The light emitted from eachregion is emitted outside the light-emitting element through eachoptical element. In other words, the light from the region 222B, thelight from the region 222G, and the light from the region 222R areemitted through the optical element 224B, the optical element 224G, andthe optical element 224R, respectively.

The optical elements 224B, 224G, and 224R each have a function ofselectively transmitting light of a particular color out of incidentlight. For example, the light emitted from the region 222B through theoptical element 224B is blue light, the light emitted from the region222G through the optical element 224G is green light, and the lightemitted from the region 222R through the optical element 224R is redlight.

Note that in FIGS. 6A and 6B, blue light (B), green light (G), and redlight (R) emitted from the regions through the optical elements areschematically illustrated by arrows of dashed lines.

A light-blocking layer 223 is provided between the optical elements. Thelight-blocking layer 223 has a function of blocking light emitted fromthe adjacent regions. Note that the light-blocking layer 223 may beomitted.

<<Microcavity>>

Furthermore, the light-emitting elements 252 and 254 each have amicrocavity structure.

Light emitted from the light-emitting layers 160 and 170 resonatesbetween a pair of electrodes (e.g., the electrodes 101 and 102). In eachof the light-emitting elements 252 and 254, the thicknesses of theconductive layers (the conductive layer 101 b, the conductive layer 103b, and the conductive layer 104 b) in each region are adjusted so thatthe wavelength of light emitted from the light-emitting layers 160 and170 can be intensified. Note that the thickness of at least one of thehole-injection layer 111 and the hole-transport layer 112 may differbetween the regions so that the wavelength of light emitted from thelight-emitting layers 160 and 170 is intensified.

For example, in the case where the refractive index of the conductivematerial having a function of reflecting light in the electrodes 101 to104 is lower than the refractive index of the light-emitting layer 160or 170, the thickness of the conductive layer 101 b of the electrode 101is adjusted so that the optical path length between the electrode 101and the electrode 102 is m_(B)λ_(B)/2 (m_(B) is a natural number andλ_(B) is a wavelength of light which is intensified in the region 222B).Similarly, the thickness of the conductive layer 103 b of the electrode103 is adjusted so that the optical path length between the electrode103 and the electrode 102 is m_(G)λ_(G)/2 (m_(G) is a natural number andλ_(G) is a wavelength of light which is intensified in the region 222G).Furthermore, the thickness of the conductive layer 104 b of theelectrode 104 is adjusted so that the optical path length between theelectrode 104 and the electrode 102 is m_(R)λ_(R)/2 (m_(R) is a naturalnumber and λ_(R) is a wavelength of light which is intensified in theregion 222R).

In the above manner, with the microcavity structure, in which theoptical path length between the pair of electrodes in the respectiveregions is adjusted, scattering and absorption of light in the vicinityof the electrodes can be suppressed, resulting in high outcouplingefficiency. In the above structure, each of the conductive layers 101 b,103 b, and 104 b preferably has a function of transmitting light. Thematerials of the conductive layers 101 b, 103 b, and 104 b may be thesame or different. The conductive layers 101 b, 103 b, and 104 b mayeach have two or more stacked layers.

Note that since the light-emitting element 252 illustrated in FIG. 6Ahas a top-emission structure, it is preferable that the conductive layer101 a of the electrode 101, the conductive layer 103 a of the electrode103, and the conductive layer 104 a of the electrode 104 have a functionof reflecting light. In addition, it is preferable that the electrode102 have functions of transmitting light and reflecting light.

Since the light-emitting element 254 illustrated in FIG. 6B has abottom-emission structure, it is preferable that the conductive layer101 a of the electrode 101, the conductive layer 103 a of the electrode103, and the conductive layer 104 a of the electrode 104 have functionsof transmitting light and reflecting light. In addition, it ispreferable that the electrode 102 have a function of reflecting light.

Materials used for the conductive layers 101 a, 103 a, and 104 a may bethe same or different in each of the light-emitting elements 252 and254. When the conductive layers 101 a, 103 a, and 104 a are formed usingthe same materials, manufacturing costs of the light-emitting elements252 and 254 can be reduced. The conductive layers 101 a, 103 a, and 104a may each have two or more stacked layers.

As described in Embodiment 1, the LUMO level of the material used in theelectron-transport layer 113 is made lower than the LUMO level of thehost material contained in the light-emitting layer 170, or the LUMOlevel of the material used in the electron-transport layer 118 is madelower than the LUMO level of the host material contained in thelight-emitting layer 160. This allows the fabrication of alight-emitting element in which a delayed fluorescence componentaccounts for a relatively high proportion of light emission from thelight-emitting layer.

The light-emitting layers 160 and 170 can each have a stacked-layerstructure of two layers, for example, a light-emitting layer 170 a and alight-emitting layer 170 b. By using two kinds of light-emittingmaterials (a first compound and a second compound) having functions ofemitting light of different colors in the two light-emitting layers,light of a plurality of emission colors can be obtained at the sametime. It is particularly preferable to select light-emitting materialsso that white light can be obtained by combining light emission from thelight-emitting layers 160 and 170.

The light-emitting layer 160 or 170 may have a structure in which threeor more layers are stacked or may include a layer containing nolight-emitting material.

The structure described in this embodiment can be combined with any ofthe structures described in the other embodiments as appropriate.

Embodiment 3

In this embodiment, light-emitting elements having structures differentfrom those described in Embodiments 1 and 2 and emission mechanisms ofthe light-emitting elements will be described below with reference toFIG. 7A to FIG. 8B.

<Structure Example 1 of Light-Emitting Element>

FIG. 7A is a schematic cross-sectional view of a light-emitting element450.

The light-emitting element 450 illustrated in FIG. 7A includes aplurality of light-emitting units (a light-emitting unit 441 and alight-emitting unit 442 in FIG. 7A) between a pair of electrodes (anelectrode 401 and an electrode 402). One light-emitting unit has thesame structure as the EL layer 100 illustrated in FIG. 1A. That is, thelight-emitting element 150 in FIG. 1A includes one light-emitting unit,whereas the light-emitting element 450 includes a plurality oflight-emitting units. Note that the electrode 401 functions as an anodeand the electrode 402 functions as a cathode in the followingdescription of the light-emitting element 450; however, the functionsmay be interchanged in the light-emitting element 450.

In the light-emitting element 450 illustrated in FIG. 7A, thelight-emitting unit 441 and the light-emitting unit 442 are stacked, anda charge-generation layer 445 is provided between the light-emittingunit 441 and the light-emitting unit 442. Note that the light-emittingunit 441 and the light-emitting unit 442 may have the same structure ordifferent structures. For example, the EL layer 100 illustrated in FIG.1A is preferably used in the light-emitting unit 441.

That is, the light-emitting element 450 includes a light-emitting layer420 and a light-emitting layer 430. The light-emitting unit 441 includesa hole-injection layer 411, a hole-transport layer 412, anelectron-transport layer 413, and an electron-injection layer 414 inaddition to the light-emitting layer 420. The light-emitting unit 442includes a hole-injection layer 416, a hole-transport layer 417, anelectron-transport layer 418, and an electron-injection layer 419 inaddition to the light-emitting layer 430.

The charge-generation layer 445 contains a composite material of anorganic compound and an acceptor substance. For the composite material,the composite material that can be used for the hole-injection layer 111described in Embodiment 1 may be used. As the organic compound, avariety of compounds such as an aromatic amine compound, a carbazolecompound, an aromatic hydrocarbon, and a high molecular compound (suchas an oligomer, a dendrimer, or a polymer) can be used. An organiccompound having a hole mobility of 1×10⁻⁶ cm²/Vs or higher is preferablyused. Note that any other material may be used as long as it has aproperty of transporting more holes than electrons. Since the compositematerial of an organic compound and an acceptor substance has excellentcarrier-injection and carrier-transport properties, low-voltage drivingor low-current driving can be realized. Note that when a surface of alight-emitting unit on the anode side is in contact with thecharge-generation layer 445 as that of the light-emitting unit 442, thecharge-generation layer 445 can also serve as a hole-injection layer ora hole-transport layer of the light-emitting unit; thus, ahole-injection layer or a hole-transport layer does need not be includedin the light-emitting unit.

The charge-generation layer 445 may have a stacked-layer structure of alayer containing the composite material of an organic compound and anacceptor substance and a layer containing another material. For example,the charge-generation layer 445 may be formed using a combination of alayer containing the composite material of an organic compound and anacceptor substance with a layer containing one compound selected fromamong materials having an electron donating property and a compoundhaving a high electron-transport property. Furthermore, thecharge-generation layer 445 may be formed using a combination of a layercontaining the composite material of an organic compound and an acceptorsubstance with a layer including a transparent conductive film.

The charge-generation layer 445 provided between the light-emitting unit441 and the light-emitting unit 442 may have any structure as long aselectrons can be injected to the light-emitting unit on one side andholes can be injected into the light-emitting unit on the other sidewhen a voltage is applied between the electrode 401 and the electrode402. For example, in FIG. 7A, the charge-generation layer 445 injectselectrons into the light-emitting unit 441 and holes into thelight-emitting unit 442 when a voltage is applied such that thepotential of the electrode 401 is higher than that of the electrode 402.

Although the light-emitting element in FIG. 7A includes the twolight-emitting units, the light-emitting element may include three ormore light-emitting units stacked. When a plurality of light-emittingunits partitioned by the charge-generation layer are arranged between apair of electrodes as in the light-emitting element 450, ahigh-luminance light-emitting element with a long lifetime can beachieved while the current density is kept low. A light-emitting elementwith low power consumption can also be provided.

The light-emitting layer 420 includes a host material 421 and a guestmaterial 422. The light-emitting layer 430 includes a host material 431and a guest material 432. The host material 431 includes an organiccompound 431_1 and an organic compound 431_2.

In this embodiment, the light-emitting layer 420 has a structure similarto that of the light-emitting layer 130 in FIG. 1A. That is, the hostmaterial 421 and the guest material 422 in the light-emitting layer 420correspond to the host material 131 and the guest material 132,respectively, in the light-emitting layer 130. In the followingdescription, the guest material 432 contained in the light-emittinglayer 430 is a phosphorescent material. Note that the electrode 401, theelectrode 402, the hole-injection layers 411 and 416, the hole-transportlayers 412 and 417, the electron-transport layers 413 and 418, and theelectron-injection layers 414 and 419 correspond to the electrode 101,the electrode 102, the hole-injection layer 111, the hole-transportlayer 112, the electron-transport layer 118, and the electron-injectionlayer 119 in Embodiment 1, respectively. Therefore, detailed descriptionthereof is omitted in this embodiment.

As described in Embodiment 1, the LUMO level of the material used in theelectron-transport layer 413 is made lower than the LUMO level of thehost material contained in the light-emitting layer 420, or the LUMOlevel of the material used in the electron-transport layer 418 is madelower than the LUMO level of the host material contained in thelight-emitting layer 430. This allows the fabrication of alight-emitting element in which a delayed fluorescence componentaccounts for a relatively high proportion of light emission from thelight-emitting layer.

<<Emission mechanism of light-emitting layer 420>>

The emission mechanism of the light-emitting layer 420 is similar tothat of the light-emitting layer 130 in FIG. 1A.

<<Emission Mechanism of Light-Emitting Layer 430>>

Next, the emission mechanism of the light-emitting layer 430 will bedescribed below.

The organic compound 431_1 and the organic compound 431_2 which arecontained in the light-emitting layer 430 form an exciplex. The organiccompound 431_1 serves as a host material and the organic compound 431_2serves as an assist material in the description here.

Although it is acceptable as long as the combination of the organiccompound 431_1 and the organic compound 431_2 can form an exciplex inthe light-emitting layer 430, it is preferred that one organic compoundbe a material having a hole-transport property and the other organiccompound be a material having an electron-transport property.

FIG. 7B illustrates the correlation of energy levels of the organiccompound 431_1, the organic compound 431_2, and the guest material 432in the light-emitting layer 430. The following explains what terms andsigns in FIG. 7B represent:

Host (431_1): the organic compound 431_1 (host material);

Assist (431_2): the organic compound 431_2 (assist material);

Guest (432): the guest material 432 (phosphorescent material);

Exciplex: exciplex

S_(PH): the level of the lowest singlet excited state of the organiccompound 431_1;

T_(PH): the level of the lowest triplet excited state of the organiccompound 431_1;

T_(PG): the level of the lowest triplet excited state of the guestmaterial 432 (phosphorescent material);

S_(E): the level of the lowest singlet excited state of the exciplex;and

T_(E): the level of the lowest triplet excited state of the exciplex.

The level (S_(E)) of the lowest singlet excited state of the exciplex,which is formed by the organic compound 431_1 and the organic compound431_2 and the level (T_(E)) of the lowest triplet excited state of theexciplex are close to each other (see Route C in FIG. 7B).

Both energies of S_(E) and T_(E) of the exciplex are then transferred tothe level of the lowest triplet excited state of the guest material 432(phosphorescent material), so that light emission is obtained (see RouteD in FIG. 7B).

The above-described processes through Route C and Route D may bereferred to as exciplex-triplet energy transfer (ExTET) in thisspecification and the like.

When one of the organic compounds 431_1 and 431_2 receiving holes andthe other receiving electrons come close to each other, the exciplex isformed at once. Alternatively, when one compound is brought into anexcited state, the one immediately interacts with the other compound toform the exciplex. Therefore, most excitons in the light-emitting layer430 exist as exciplexes. The band gap of the exciplex is narrower thanthat of each of the organic compounds 431_1 and 431_2; therefore, thedriving voltage can be lowered when the exciplex is formed byrecombination of a hole and an electron.

When the light-emitting layer 430 has the above structure, lightemission from the guest material 432 (phosphorescent material) of thelight-emitting layer 430 can be efficiently obtained.

Note that light emitted from the light-emitting layer 420 preferably hasa peak on the shorter wavelength side than light emitted from thelight-emitting layer 430. The luminance of a light-emitting elementusing the phosphorescent material emitting light with a short wavelengthtends to degrade quickly. By using fluorescence for light emission witha short wavelength, a light-emitting element with less degradation ofluminance can be provided.

Furthermore, the light-emitting layer 420 and the light-emitting layer430 may be made to emit light with different emission wavelengths, sothat the light-emitting element can be a multicolor light-emittingelement. In that case, the emission spectrum of the light-emittingelement is formed by combining light having different emission peaks,and thus has at least two peaks.

The above structure is also suitable for obtaining white light emission.When the light-emitting layer 420 and the light-emitting layer 430 emitlight of complementary colors, white light emission can be obtained.

In addition, white light emission with a high color rendering propertythat is formed of three primary colors or four or more colors can beobtained by using a plurality of light-emitting materials emitting lightwith different wavelengths for one of the light-emitting layers 420 and430 or both. In that case, one of the light-emitting layers 420 and 430or both may be divided into layers and each of the divided layers maycontain a different light-emitting material from the others.

<Examples of Material that can be Used for Light-Emitting Layer>

Next, materials that can be used for the light-emitting layers 420 and430 will be described.

<<Material that can be Used for Light-Emitting Layer 420>>

Any of the materials that can be used for the light-emitting layer 130described in Embodiment 1 may be used as a material that can be used forthe light-emitting layer 420.

<<Material that can be Used for Light-Emitting Layer 430>>

In the light-emitting layer 430, the organic compound 431_1 (hostmaterial) exists in the highest proportion in weight ratio, and theguest material 432 (phosphorescent material) is dispersed in the organiccompound 431_1 (host material).

Examples of the organic compound 431_1 (host material) include a zinc-or aluminum-based metal complex, an oxadiazole derivative, a triazolederivative, a benzimidazole derivative, a quinoxaline derivative, adibenzoquinoxaline derivative, a dibenzothiophene derivative, adibenzofuran derivative, a pyrimidine derivative, a triazine derivative,a pyridine derivative, a bipyridine derivative, and a phenanthrolinederivative. Other examples are an aromatic amine, a carbazolederivative, and the like.

As the guest material 432 (phosphorescent material), an iridium-,rhodium-, or platinum-based organometallic complex or metal complex canbe used; in particular, an organoiridium complex such as aniridium-based ortho-metalated complex is preferable. As anortho-metalated ligand, a 4H-triazole ligand, a 1H-triazole ligand, animidazole ligand, a pyridine ligand, a pyrimidine ligand, a pyrazineligand, an isoquinoline ligand, and the like can be given. As the metalcomplex, a platinum complex having a porphyrin ligand, and the like canbe given.

As the organic compound 431_2 (assist material), a substance which canform an exciplex together with the organic compound 431_1 is used. Inthat case, it is preferable that the organic compound 431_1, the organiccompound 431_2, and the guest material 432 (phosphorescent material) beselected such that the emission peak of the exciplex overlaps with anabsorption band, specifically an absorption band on the longestwavelength side, of a triplet metal to ligand charge transfer (MLCT)transition of the guest material 432 (phosphorescent material). Thismakes it possible to provide a light-emitting element with drasticallyimproved emission efficiency. Note that in the case where a thermallyactivated delayed fluorescence material is used instead of thephosphorescent material, it is preferable that the absorption band onthe longest wavelength side be a singlet absorption band.

As the light-emitting material contained in the light-emitting layer430, any material can be used as long as the material can converttriplet excitation energy into light emission. As an example of thematerial that can convert triplet excitation energy into light emission,a thermally activated delayed fluorescence (TADF) material can be givenin addition to the phosphorescent material. Therefore, the term“phosphorescent material” in the description can be replaced with theterm “thermally activated delayed fluorescence material”. Note that thethermally activated delayed fluorescence material is a material that canup-convert a triplet excited state into a singlet excited state (i.e.,reverse intersystem crossing is possible) using a little thermal energyand efficiently exhibits light emission (fluorescence) from the singletexcited state. Thermally activated delayed fluorescence is efficientlyobtained under the conditions where the difference between the tripletexcitation energy level and the singlet excitation energy level ispreferably larger than 0 eV and smaller than or equal to 0.2 eV, furtherpreferably larger than 0 eV and smaller than or equal to 0.1 eV.

The material that emits thermally activated delayed fluorescence may bea material that can form a singlet excited state by itself from atriplet excited state by reverse intersystem crossing or may be acombination of two kinds of materials which form an exciplex.

There is no limitation on the emission colors of the light-emittingmaterials contained in the light-emitting layers 420 and 430, and theymay be the same or different. Light emitted from the light-emittingmaterials is mixed and extracted out of the element; therefore, forexample, in the case where their emission colors are complementarycolors, the light-emitting element can emit white light. Inconsideration of the reliability of the light-emitting element, theemission peak wavelength of the light-emitting material contained in thelight-emitting layer 420 is preferably shorter than that of thelight-emitting material contained in the light-emitting layer 430.

<Structure Example 2 of Light-Emitting Element>

Next, a structure example different from that of the light-emittingelement illustrated in FIGS. 7A and 7B will be described below withreference to FIGS. 8A and 8B.

FIG. 8A is a schematic cross-sectional view of a light-emitting element452.

In the light-emitting element 452 in FIG. 8A, an EL layer 400 isinterposed between a pair of electrodes (the electrodes 401 and 402).Note that in the light-emitting element 452, the electrode 401 functionsas an anode, and the electrode 402 functions as a cathode.

The EL layer 400 includes the light-emitting layers 420 and 430. As theEL layer 400 in the light-emitting element 452, the light-emittinglayers 420 and 430, the hole-injection layer 411, the hole-transportlayer 412, the electron-transport layer 418, and the electron-injectionlayer 419 are illustrated. However, this stacked-layer structure is anexample, and the structure of the EL layer 400 in the light-emittingelement 452 is not limited thereto. For example, the stacking order ofthe above layers of the EL layer 400 may be changed. Alternatively, inthe EL layer 400, a functional layer other than the above layers may beprovided. The functional layer may have a function of injecting acarrier (an electron or a hole), a function of transporting a carrier, afunction of inhibiting a carrier, or a function of generating a carrier,for example.

The light-emitting layer 420 includes the host material 421 and theguest material 422. The light-emitting layer 430 includes the hostmaterial 431 and the guest material 432. The host material 431 includesthe organic compound 431_1 and the organic compound 431_2. In thefollowing description, the guest material 422 is a fluorescent materialand the guest material 432 is a phosphorescent material.

<<Emission Mechanism of Light-Emitting Layer 420>>

The emission mechanism of the light-emitting layer 420 is similar tothat of the light-emitting layer 130 in FIG. 1A.

<<Emission Mechanism of Light-Emitting Layer 430>>

The emission mechanism of the light-emitting layer 430 is similar tothat of the light-emitting layer 430 in FIG. 7A.

<<Emission Mechanism of Light-Emitting Layers 420 and 430>>

Each emission mechanism of the light-emitting layers 420 and 430 isdescribed above. As in the light-emitting element 452, in the case wherethe light-emitting layers 420 and 430 are in contact with each other,even when energy is transferred from the exciplex to the host material421 of the light-emitting layer 420 (in particular, when energy of thetriplet excited level is transferred) at an interface between thelight-emitting layer 420 and the light-emitting layer 430, the tripletexcitation energy can be converted into light emission in thelight-emitting layer 420.

The T1 level of the host material 421 of the light-emitting layer 420 ispreferably lower than T1 levels of the organic compounds 431_1 and 431_2in the light-emitting layer 430. In the light-emitting layer 420, the S1level of the host material 421 is preferably higher than the S1 level ofthe guest material 422 (fluorescent material) while the T1 level of thehost material 421 is preferably lower than the T1 level of the guestmaterial 422 (fluorescent material).

FIG. 8B shows the correlation of energy levels in the case where TTA isutilized in the light-emitting layer 420 and ExTET is utilized in thelight-emitting layer 430. The following explains what terms and signs inFIG. 8B represent:

Fluorescence EML (420): the fluorescent light-emitting layer(light-emitting layer 420);

Phosphorescence EML (430): the phosphorescent light-emitting layer(light-emitting layer 430);

S_(FH): the level of the lowest singlet excited state of the hostmaterial 421;

T_(FH): the level of the lowest triplet excited state of the hostmaterial 421;

S_(FG): the level of the lowest singlet excited state of the guestmaterial 422 (fluorescent material);

T_(FG): the level of the lowest triplet excited state of the guestmaterial 422 (fluorescent material);

S_(PH): the level of the lowest singlet excited state of the hostmaterial (organic compound 431_1);

T_(PH): the level of the lowest triplet excited state of the hostmaterial (organic compound 431_1);

T_(PG): the level of the lowest triplet excited state of the guestmaterial 432 (phosphorescent material);

S_(E): the level of the lowest singlet excited state of the exciplex;and

T_(E): the level of the lowest triplet excited state of the exciplex.

As shown in FIG. 8B, the exciplex exists only in an excited state; thus,exciton diffusion between the exciplexes is less likely to occur. Inaddition, because the excited levels (S_(E) and T_(E)) of the exciplexare lower than the excited levels (S_(PH) and T_(PH)) of the organiccompound 431_1 (the host material for the phosphorescent material) ofthe light-emitting layer 430, energy diffusion from the exciplex to theorganic compound 431_1 does not occur. Similarly, energy diffusion fromthe exciplex to the organic compound 431_2 does not occur. That is, theefficiency of the phosphorescent light-emitting layer (light-emittinglayer 430) can be maintained because an exciton diffusion distance ofthe exciplex is short in the phosphorescent light-emitting layer(light-emitting layer 430). In addition, even when part of the tripletexcitation energy of the exciplex of the phosphorescent light-emittinglayer (light-emitting layer 430) diffuses into the fluorescentlight-emitting layer (light-emitting layer 420) through the interfacebetween the fluorescent light-emitting layer (light-emitting layer 420)and the phosphorescent light-emitting layer (light-emitting layer 430),energy loss can be reduced because the triplet excitation energy in thefluorescent light-emitting layer (light-emitting layer 420) caused bythe diffusion is used for light emission through TTA.

As described above, ExTET is utilized in the light-emitting layer 430and TTA is utilized in the light-emitting layer 420 in thelight-emitting element 452, so that energy loss is reduced and a highemission efficiency is achieved. As in the light-emitting element 452,in the case where the light-emitting layers 420 and 430 are in contactwith each other, the number of EL layers 400 as well as the energy losscan be reduced. Therefore, a light-emitting element with lowmanufacturing costs can be obtained.

Note that the light-emitting layers 420 and 430 need not be in contactwith each other. In that case, it is possible to prevent energy transferby the Dexter mechanism (particularly triplet energy transfer) from theorganic compound 431_1 or 431_2 in an excited state or the guestmaterial 432 (phosphorescent material) in an excited state which isgenerated in the light-emitting layer 430 to the host material 421 orthe guest material 422 (fluorescent material) in the light-emittinglayer 420. Therefore, the thickness of a layer provided between thelight-emitting layers 420 and 430 may be several nanometers.

The layer provided between the light-emitting layers 420 and 430 maycontain a single material or both a hole-transport material and anelectron-transport material. In the case of a single material, a bipolarmaterial may be used. The bipolar material here refers to a material inwhich the ratio between the electron mobility and the hole mobility is100 or less. Alternatively, the hole-transport material, theelectron-transport material, or the like may be used. At least one ofmaterials included in the layer may be the same as the host material(organic compound 431_1 or 431_2) of the light-emitting layer 430. Thisfacilitates the manufacture of the light-emitting element and reducesthe drive voltage. Furthermore, the hole-transport material and theelectron-transport material may form an exciplex, which effectivelyprevents exciton diffusion. Specifically, it is possible to preventenergy transfer from the host material (organic compound 431_1 or 431_2)in an excited state or the guest material 432 (phosphorescent material)in an excited state of the light-emitting layer 430 to the host material421 or the guest material 422 (fluorescent material) in thelight-emitting layer 420.

Note that in the light-emitting element 452, a carrier recombinationregion is preferably distributed to some extent. Therefore, it ispreferable that the light-emitting layer 420 or 430 have an appropriatedegree of carrier-trapping property. It is particularly preferable thatthe guest material 432 (phosphorescent material) in the light-emittinglayer 430 have an electron-trapping property. Alternatively, the guestmaterial 422 (fluorescent material) in the light-emitting layer 420preferably has a hole-trapping property.

Note that light emitted from the light-emitting layer 420 preferably hasa peak on the shorter wavelength side than light emitted from thelight-emitting layer 430. Since the luminance of a light-emittingelement using a phosphorescent material emitting light with a shortwavelength tends to degrade quickly, fluorescence with a shortwavelength is employed so that a light-emitting element with lessdegradation of luminance can be provided.

Furthermore, when the light-emitting layers 420 and 430 are made to emitlight with different emission wavelengths, a multicolor light-emittingelement can be achieved. In that case, the emission spectrum of thelight-emitting element is formed by combining light having differentemission peaks, and thus has at least two peaks.

The above structure is also suitable for obtaining white light emission.When the light-emitting layers 420 and 430 emit light of complementarycolors, white light emission can be obtained.

In addition, white light emission with a high color rendering propertythat is formed of three primary colors or four or more colors can beobtained by using a plurality of light-emitting substances emittinglight with different wavelengths for the light-emitting layer 420. Inthat case, the light-emitting layer 420 may be divided into layers andeach of the divided layers may contain a light-emitting materialdifferent from the others.

<Material that can be Used in Light-Emitting Layer>

Next, materials that can be used in the light-emitting layers 420 and430 will be described.

<<Material that can be Used in Light-Emitting Layer 420>>

In the light-emitting layer 420, the host material 421 is present in thehighest proportion by weight, and the guest material 422 (fluorescentmaterial) is dispersed in the host material 421. The S1 level of thehost material 421 is preferably higher than the S1 level of the guestmaterial 422 (fluorescent material) while the T1 level of the hostmaterial 421 is preferably lower than the T1 level of the guest material422 (fluorescent material).

<<Material that can be Used in Light-Emitting Layer 430>>

In the light-emitting layer 430, the host material (organic compound431_1 or 431_2) is present in the highest proportion by weight, and theguest material 432 (phosphorescent material) is dispersed in the hostmaterials (organic compounds 431_1 and 431_2). The T1 levels of the hostmaterials (organic compounds 431_1 and 431_2) of the light-emittinglayer 430 is preferably higher than the T1 level of the guest material422 (fluorescent material) of the light-emitting layer 420.

As the host materials (organic compounds 431_1 and 431_2) and the guestmaterial 432 (phosphorescent material), those in the light-emittingelement 450 described in FIGS. 7A and 7B can be used.

Note that the light-emitting layers 420 and 430 can be formed by anevaporation method (including a vacuum evaporation method), an inkjetmethod, a coating method, gravure printing, or the like.

The structure described in this embodiment can be combined with any ofthe structures described in the other embodiments as appropriate.

Embodiment 4

In this embodiment, a display device including a light-emitting elementof one embodiment of the present invention will be described withreference to FIGS. 9A and 9B.

Note that FIG. 9A is a block diagram illustrating the display device ofone embodiment of the present invention, and FIG. 9B is a circuitdiagram illustrating a pixel circuit of the display device of oneembodiment of the present invention.

<Description of Display Device>

The display device illustrated in FIG. 9A includes a region includingpixels of display elements (the region is hereinafter referred to as apixel portion 802), a circuit portion provided outside the pixel portion802 and including circuits for driving the pixels (the portion ishereinafter referred to as a driver circuit portion 804), circuitshaving a function of protecting elements (the circuits are hereinafterreferred to as protection circuits 806), and a terminal portion 807.Note that the protection circuits 806 are not necessarily provided.

A part or the whole of the driver circuit portion 804 is preferablyformed over a substrate over which the pixel portion 802 is formed, inwhich case the number of components and the number of terminals can bereduced. When a part or the whole of the driver circuit portion 804 isnot formed over the substrate over which the pixel portion 802 isformed, the part or the whole of the driver circuit portion 804 can bemounted by COG or tape automated bonding (TAB).

The pixel portion 802 includes a plurality of circuits for drivingdisplay elements arranged in X rows (X is a natural number of 2 or more)and Y columns (Y is a natural number of 2 or more) (such circuits arehereinafter referred to as pixel circuits 801). The driver circuitportion 804 includes driver circuits such as a circuit for supplying asignal (scan signal) to select a pixel (the circuit is hereinafterreferred to as a scan line driver circuit 804 a) and a circuit forsupplying a signal (data signal) to drive a display element in a pixel(the circuit is hereinafter referred to as a signal line driver circuit804 b).

The scan line driver circuit 804 a includes a shift register or thelike. Through the terminal portion 807, the scan line driver circuit 804a receives a signal for driving the shift register and outputs a signal.For example, the scan line driver circuit 804 a receives a start pulsesignal, a clock signal, or the like and outputs a pulse signal. The scanline driver circuit 804 a has a function of controlling the potentialsof wirings supplied with scan signals (such wirings are hereinafterreferred to as scan lines GL_1 to GL_X). Note that a plurality of scanline driver circuits 804 a may be provided to control the scan linesGL_1 to GL_X separately. Alternatively, the scan line driver circuit 804a has a function of supplying an initialization signal. Without beinglimited thereto, the scan line driver circuit 804 a can supply anothersignal.

The signal line driver circuit 804 b includes a shift register or thelike. Through the terminal portion 807, the signal line driver circuit804 b receives a signal (image signal) from which a data signal isderived, as well as a signal for driving the shift register. The signalline driver circuit 804 b has a function of generating a data signal tobe written to the pixel circuit 801 which is based on the image signal.In addition, the signal line driver circuit 804 b has a function ofcontrolling the output of a data signal in response to a pulse signalproduced by the input of a start pulse signal, a clock signal, or thelike. Furthermore, the signal line driver circuit 804 b has a functionof controlling the potentials of wirings supplied with data signals(such wirings are hereinafter referred to as data lines DL_1 to DL_Y).Alternatively, the signal line driver circuit 804 b has a function ofsupplying an initialization signal. Without being limited thereto, thesignal line driver circuit 804 b can supply another signal.

The signal line driver circuit 804 b includes a plurality of analogswitches, for example. The signal line driver circuit 804 b can output,as the data signals, signals obtained by time-dividing the image signalby sequentially turning on the plurality of analog switches. The signalline driver circuit 804 b may include a shift register or the like.

To each of the plurality of pixel circuits 801, a pulse signal is inputthrough one of the plurality of scan lines GL supplied with scan signalsand a data signal is input through one of the plurality of data lines DLsupplied with data signals. Writing and holding of the data signal toand in each of the plurality of pixel circuits 801 are controlled by thescan line driver circuit 804 a. For example, to the pixel circuit 801 inthe m-th row and the n-th column (m is a natural number of X or less,and n is a natural number of Y or less), a pulse signal is input fromthe scan line driver circuit 804 a through the scan line GL_m, and adata signal is input from the signal line driver circuit 804 b throughthe data line DL_n in accordance with the potential of the scan lineGL_m.

The protection circuit 806 shown in FIG. 9A is connected to, forexample, the scan line GL between the scan line driver circuit 804 a andthe pixel circuit 801. Alternatively, the protection circuit 806 isconnected to the data line DL between the signal line driver circuit 804b and the pixel circuit 801. Alternatively, the protection circuit 806can be connected to a wiring between the scan line driver circuit 804 aand the terminal portion 807. Alternatively, the protection circuit 806can be connected to a wiring between the signal line driver circuit 804b and the terminal portion 807. Note that the terminal portion 807 meansa portion having terminals for inputting power, control signals, andimage signals to the display device from external circuits.

The protection circuit 806 is a circuit that electrically connects awiring connected to the protection circuit to another wiring when apotential out of a certain range is applied to the wiring connected tothe protection circuit.

As illustrated in FIG. 9A, the protection circuits 806 are connected tothe pixel portion 802 and the driver circuit portion 804, so that theresistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like can be improved. Note that theconfiguration of the protection circuits 806 is not limited to that, andfor example, the protection circuits 806 may be connected to the scanline driver circuit 804 a or the signal line driver circuit 804 b.Alternatively, the protection circuits 806 may be connected to theterminal portion 807.

FIG. 9A shows an example in which the driver circuit portion 804includes the scan line driver circuit 804 a and the signal line drivercircuit 804 b; however, the structure is not limited thereto. Forexample, only the scan line driver circuit 804 a may be formed and aseparately prepared substrate where a signal line driver circuit isformed (e.g., a driver circuit substrate formed with a single crystalsemiconductor film or a polycrystalline semiconductor film) may bemounted.

<Structure Example of Pixel Circuit>

Each of the plurality of pixel circuits 801 in FIG. 9A can have astructure illustrated in FIG. 9B, for example.

The pixel circuit 801 illustrated in FIG. 9B includes transistors 852and 854, a capacitor 862, and a light-emitting element 872.

One of a source electrode and a drain electrode of the transistor 852 iselectrically connected to a wiring to which a data signal is supplied (adata line DL_n). A gate electrode of the transistor 852 is electricallyconnected to a wiring to which a gate signal is supplied (a scan lineGL_m).

The transistor 852 has a function of controlling whether to write a datasignal.

One of a pair of electrodes of the capacitor 862 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 852.

The capacitor 862 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 854 iselectrically connected to the potential supply line VL_a. Furthermore, agate electrode of the transistor 854 is electrically connected to theother of the source electrode and the drain electrode of the transistor852.

One of an anode and a cathode of the light-emitting element 872 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 854.

As the light-emitting element 872, any of the light-emitting elementsdescribed in Embodiments 1 to 3 can be used.

Note that a high power supply potential VDD is supplied to one of thepotential supply line VL_a and the potential supply line VL_b, and a lowpower supply potential VSS is supplied to the other.

In the display device including the pixel circuits 801 in FIG. 9B, forexample, the pixel circuits 801 are sequentially selected row by row bythe scan line driver circuit 804 a in FIG. 9A, whereby the transistors852 are turned on and a data signal is written.

When the transistors 852 are turned off, the pixel circuits 801 in whichthe data has been written are brought into a holding state. Furthermore,the amount of current flowing between the source electrode and the drainelectrode of the transistor 854 is controlled in accordance with thepotential of the written data signal. The light-emitting element 872emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage is displayed.

A light-emitting element of one embodiment of the present invention canbe used for an active matrix method in which an active element isincluded in a pixel of a display device or a passive matrix method inwhich an active element is not included in a pixel of a display device.

In the active matrix method, as an active element (a non-linearelement), not only a transistor but also a variety of active elements(non-linear elements) can be used. For example, a metal insulator metal(MIM), a thin film diode (TFD), or the like can also be used. Sincethese elements can be formed with a smaller number of manufacturingsteps, manufacturing costs can be reduced or yield can be improved.Alternatively, since the size of these elements is small, the apertureratio can be improved, so that power consumption can be reduced andhigher luminance can be achieved.

As a method other than the active matrix method, the passive matrixmethod in which an active element (a non-linear element) is not used canalso be used. Since an active element (a non-linear element) is notused, the number of manufacturing steps is small, so that manufacturingcosts can be reduced or yield can be improved. Alternatively, since anactive element (a non-linear element) is not used, the aperture ratiocan be improved, so that power consumption can be reduced or higherluminance can be achieved, for example.

The structure described in this embodiment can be combined with any ofthe structures described in the other embodiments as appropriate.

Embodiment 5

In this embodiment, a display device including a light-emitting elementof one embodiment of the present invention and an electronic device inwhich the display device is provided with an input device will bedescribed with reference to FIGS. 10A to 14.

<Description 1 of Touch Panel>

In this embodiment, a touch panel 2000 including a display device and aninput device will be described as an example of an electronic device. Inaddition, an example of using a touch sensor as an input device will bedescribed.

FIGS. 10A and 10B are perspective views of the touch panel 2000. Notethat FIGS. 10A and 10B illustrate typical components of the touch panel2000 for simplicity.

The touch panel 2000 includes a display device 2501 and a touch sensor2595 (see FIG. 10B). The touch panel 2000 also includes a substrate2510, a substrate 2570, and a substrate 2590. The substrate 2510, thesubstrate 2570, and the substrate 2590 each have flexibility. Note thatone or all of the substrates 2510, 2570, and 2590 may be inflexible.

The display device 2501 includes a plurality of pixels over thesubstrate 2510 and a plurality of wirings 2511 through which signals aresupplied to the pixels. The plurality of wirings 2511 are led to aperipheral portion of the substrate 2510, and parts of the plurality ofwirings 2511 form a terminal 2519. The terminal 2519 is electricallyconnected to an FPC 2509(1).

The substrate 2590 includes the touch sensor 2595 and a plurality ofwirings 2598 electrically connected to the touch sensor 2595. Theplurality of wirings 2598 are led to a peripheral portion of thesubstrate 2590, and parts of the plurality of wirings 2598 form aterminal. The terminal is electrically connected to an FPC 2509(2). Notethat in FIG. 10B, electrodes, wirings, and the like of the touch sensor2595 provided on the back side of the substrate 2590 (the side facingthe substrate 2510) are indicated by solid lines for clarity.

As the touch sensor 2595, a capacitive touch sensor can be used, forexample. Examples of the capacitive touch sensor are a surfacecapacitive touch sensor and a projected capacitive touch sensor.

Examples of the projected capacitive touch sensor are a self capacitivetouch sensor and a mutual capacitive touch sensor, which differ mainlyin the driving method. The use of the mutual capacitive type ispreferable because multiple points can be sensed simultaneously.

Note that the touch sensor 2595 illustrated in FIG. 10B is an example ofusing a projected capacitive touch sensor.

Note that a variety of sensors that can sense proximity or touch of asensing target such as a finger can be used as the touch sensor 2595.

The projected capacitive touch sensor 2595 includes electrodes 2591 andelectrodes 2592. The electrodes 2591 are electrically connected to anyof the plurality of wirings 2598, and the electrodes 2592 areelectrically connected to any of the other wirings 2598.

The electrodes 2592 have a shape of a plurality of quadrangles arrangedin one direction with one corner of a quadrangle connected to one cornerof another quadrangle as illustrated in FIGS. 10A and 10B.

The electrodes 2591 each have a quadrangular shape and are arranged in adirection intersecting with the direction in which the electrodes 2592extend.

A wiring 2594 electrically connects two electrodes 2591 between whichthe electrode 2592 is positioned. The intersecting area of the electrode2592 and the wiring 2594 is preferably as small as possible. Such astructure allows a reduction in the area of a region where theelectrodes are not provided, reducing variation in transmittance. As aresult, variation in luminance of light passing through the touch sensor2595 can be reduced.

Note that the shapes of the electrodes 2591 and the electrodes 2592 arenot limited thereto and can be any of a variety of shapes. For example,a structure may be employed in which the plurality of electrodes 2591are arranged so that gaps between the electrodes 2591 are reduced asmuch as possible, and the electrodes 2592 are spaced apart from theelectrodes 2591 with an insulating layer interposed therebetween to haveregions not overlapping with the electrodes 2591. In this case, it ispreferable to provide, between two adjacent electrodes 2592, a dummyelectrode electrically insulated from these electrodes because the areaof regions having different transmittances can be reduced.

<Description of Display Device>

Next, the display device 2501 will be described in detail with referenceto FIG. 11A. FIG. 11A corresponds to a cross-sectional view taken alongdashed-dotted line X1-X2 in FIG. 10B.

The display device 2501 includes a plurality of pixels arranged in amatrix. Each of the pixels includes a display element and a pixelcircuit for driving the display element.

An example in which a light-emitting element that emits white light isused as a display element will be described below; however, the displayelement is not limited to such an element. For example, light-emittingelements that emit light of different colors may be included so that thelight of different colors can be emitted from adjacent pixels.

For the substrate 2510 and the substrate 2570, for example, a flexiblematerial with a vapor permeability of lower than or equal to 1×10⁻⁵g·m⁻²·day⁻¹, preferably lower than or equal to 1×10⁻⁶ g·m⁻²·day⁻¹ can befavorably used. Alternatively, materials whose thermal expansioncoefficients are substantially equal to each other are preferably usedfor the substrate 2510 and the substrate 2570. For example, thecoefficients of linear expansion of the materials are preferably lowerthan or equal to 1×10⁻³/K, further preferably lower than or equal to5×10⁻⁵/K, and still further preferably lower than or equal to 1×10⁻⁵/K.

Note that the substrate 2510 is a stacked body including an insulatinglayer 2510 a for preventing impurity diffusion into the light-emittingelement, a flexible substrate 2510 b, and an adhesive layer 2510 c forattaching the insulating layer 2510 a and the flexible substrate 2510 bto each other. The substrate 2570 is a stacked body including aninsulating layer 2570 a for preventing impurity diffusion into thelight-emitting element, a flexible substrate 2570 b, and an adhesivelayer 2570 c for attaching the insulating layer 2570 a and the flexiblesubstrate 2570 b to each other.

For the adhesive layer 2510 c and the adhesive layer 2570 c, forexample, polyester, polyolefin, polyamide (e.g., nylon, aramid),polyimide, polycarbonate, or an acrylic resin, polyurethane, or an epoxyresin can be used. Alternatively, a material that includes a resinhaving a siloxane bond such as silicone can be used.

A sealing layer 2560 is provided between the substrate 2510 and thesubstrate 2570. The sealing layer 2560 preferably has a refractive indexhigher than that of air. In the case where light is extracted to thesealing layer 2560 side as illustrated in FIG. 11A, the sealing layer2560 can also serve as an optical adhesive layer.

A sealant may be formed in the peripheral portion of the sealing layer2560. With the use of the sealant, a light-emitting element 2550R can beprovided in a region surrounded by the substrate 2510, the substrate2570, the sealing layer 2560, and the sealant. Note that an inert gas(such as nitrogen and argon) may be used instead of the sealing layer2560. A drying agent may be provided in the inert gas so as to adsorbmoisture or the like. An ultraviolet curable resin or a heat curableresin may be used; for example, a polyvinyl chloride (PVC) based resin,an acrylic resin, a polyimide-based resin, an epoxy-based resin, asilicone-based resin, a polyvinyl butyral (PVB) based resin, or anethylene vinyl acetate (EVA) based resin can be used. An epoxy-basedresin or a glass frit is preferably used as the sealant. As a materialused for the sealant, a material which is impermeable to moisture andoxygen is preferably used.

The display device 2501 includes a pixel 2502R. The pixel 2502R includesa light-emitting module 2580R.

The pixel 2502R includes the light-emitting element 2550R and atransistor 2502 t that can supply electric power to the light-emittingelement 2550R. Note that the transistor 2502 t functions as part of thepixel circuit. The light-emitting module 2580R includes thelight-emitting element 2550R and a coloring layer 2567R.

The light-emitting element 2550R includes a lower electrode, an upperelectrode, and an EL layer between the lower electrode and the upperelectrode. As the light-emitting element 2550R, for example, any of thelight-emitting elements described in Embodiments 1 to 4 can be used.

A microcavity structure may be employed between the lower electrode andthe upper electrode so as to increase the intensity of light having aspecific wavelength.

In the case where the sealing layer 2560 is provided on the lightextraction side, the sealing layer 2560 is in contact with thelight-emitting element 2550R and the coloring layer 2567R.

The coloring layer 2567R is positioned in a region overlapping with thelight-emitting element 2550R. Accordingly, part of light emitted fromthe light-emitting element 2550R passes through the coloring layer 2567Rand is emitted to the outside of the light-emitting module 2580R asindicated by an arrow in the drawing.

The display device 2501 includes a light-blocking layer 2567BM on thelight extraction side. The light-blocking layer 2567BM is provided so asto surround the coloring layer 2567R.

The coloring layer 2567R is a coloring layer having a function oftransmitting light in a particular wavelength region. For example, acolor filter for transmitting light in a red wavelength region, a colorfilter for transmitting light in a green wavelength region, a colorfilter for transmitting light in a blue wavelength region, a colorfilter for transmitting light in a yellow wavelength region, or the likecan be used. Each color filter can be formed with any of variousmaterials by a printing method, an inkjet method, an etching methodusing a photolithography technique, or the like.

An insulating layer 2521 is provided in the display device 2501. Theinsulating layer 2521 covers the transistor 2502 t. Note that theinsulating layer 2521 has a function of covering unevenness caused bythe pixel circuit. The insulating layer 2521 may have a function ofsuppressing impurity diffusion. This can prevent the reliability of thetransistor 2502 t or the like from being lowered by impurity diffusion.

The light-emitting element 2550R is formed over the insulating layer2521. A partition wall 2528 is provided so as to overlap with an endportion of the lower electrode of the light-emitting element 2550R. Notethat a spacer for controlling the distance between the substrate 2510and the substrate 2570 may be formed over the partition wall 2528.

A scan line driver circuit 2503 g(1) includes a transistor 2503 t and acapacitor 2503 c. Note that the driver circuit and the pixel circuitscan be formed in the same process and over the same substrate.

The wirings 2511 through which signals can be supplied are provided overthe substrate 2510. The terminal 2519 is provided over the wirings 2511.The FPC 2509(1) is electrically connected to the terminal 2519. The FPC2509(1) has a function of supplying a video signal, a clock signal, astart signal, a reset signal, or the like. Note that the FPC 2509(1) maybe provided with a printed wiring board (PWB).

In the display device 2501, transistors with any of a variety ofstructures can be used. FIG. 11A illustrates an example of usingbottom-gate transistors; however, the present invention is not limitedto this example, and top-gate transistors may be used in the displaydevice 2501 as illustrated in FIG. 11B.

In addition, there is no particular limitation on the polarity of thetransistor 2502 t and the transistor 2503 t. For these transistors,n-channel and p-channel transistors may be used, or either n-channeltransistors or p-channel transistors may be used, for example.Furthermore, there is no particular limitation on the crystallinity of asemiconductor film used for the transistors 2502 t and 2503 t. Forexample, an amorphous semiconductor film or a crystalline semiconductorfilm may be used. Examples of semiconductor materials include Group 13semiconductors (e.g., a semiconductor including gallium), Group 14semiconductors (e.g., a semiconductor including silicon), compoundsemiconductors (including oxide semiconductors), and organicsemiconductors. An oxide semiconductor that has an energy gap of 2 eV ormore, preferably 2.5 eV or more, further preferably 3 eV or more ispreferably used for one of the transistors 2502 t and 2503 t or both, sothat the off-state current of the transistors can be reduced. Examplesof the oxide semiconductors include an In—Ga oxide and an In-M-Zn oxide(M represents aluminum (Al), gallium (Ga), yttrium (Y), zirconium (Zr),lanthanum (La), cerium (Ce), tin (Sn), hafnium (Hf), or neodymium (Nd)).

<Description of Touch Sensor>

Next, the touch sensor 2595 will be described in detail with referenceto FIG. 11C. FIG. 11C corresponds to a cross-sectional view taken alongdashed-dotted line X3-X4 in FIG. 10B.

The touch sensor 2595 includes the electrodes 2591 and the electrodes2592 provided in a staggered arrangement on the substrate 2590, aninsulating layer 2593 covering the electrodes 2591 and the electrodes2592, and the wiring 2594 that electrically connects the adjacentelectrodes 2591 to each other.

The electrodes 2591 and the electrodes 2592 are formed using alight-transmitting conductive material. As a light-transmittingconductive material, a conductive oxide such as indium oxide, indium tinoxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium isadded can be used. Note that a film including graphene may be used aswell. The film including graphene can be formed, for example, byreducing a film containing graphene oxide. As a reducing method, amethod with application of heat or the like can be employed.

The electrodes 2591 and the electrodes 2592 may be formed by, forexample, depositing a light-transmitting conductive material on thesubstrate 2590 by a sputtering method and then removing an unnecessaryportion by any of various pattern forming techniques such asphotolithography.

Examples of a material for the insulating layer 2593 are a resin such asan acrylic resin or an epoxy resin, a resin having a siloxane bond suchas silicone, and an inorganic insulating material such as silicon oxide,silicon oxynitride, or aluminum oxide.

Openings reaching the electrodes 2591 are formed in the insulating layer2593, and the wiring 2594 electrically connects the adjacent electrodes2591. Alight-transmitting conductive material can be favorably used asthe wiring 2594 because the aperture ratio of the touch panel can beincreased. Moreover, a material having higher conductivity than theelectrodes 2591 and 2592 can be favorably used for the wiring 2594because electric resistance can be reduced.

One electrode 2592 extends in one direction, and a plurality ofelectrodes 2592 are provided in the form of stripes. The wiring 2594intersects with the electrode 2592.

One electrode 2592 is provided between the pair of electrodes 2591. Thewiring 2594 electrically connects the pair of electrodes 2591.

Note that the plurality of electrodes 2591 are not necessarily arrangedin the direction orthogonal to one electrode 2592 and may be arranged tointersect with one electrode 2592 at an angle of more than 0 degrees andless than 90 degrees.

The wiring 2598 is electrically connected to any of the electrodes 2591and 2592. Part of the wiring 2598 functions as a terminal. For thewiring 2598, a metal material such as aluminum, gold, platinum, silver,nickel, titanium, tungsten, chromium, molybdenum, iron, cobalt, copper,or palladium or an alloy material containing any of these metalmaterials can be used.

Note that an insulating layer that covers the insulating layer 2593 andthe wiring 2594 may be provided to protect the touch sensor 2595.

A connection layer 2599 electrically connects the wiring 2598 to the FPC2509(2).

As the connection layer 2599, any of various anisotropic conductivefilms (ACF), anisotropic conductive pastes (ACP), or the like can beused.

<Description 2 of Touch Panel>

Next, the touch panel 2000 will be described in detail with reference toFIG. 12A. FIG. 12A corresponds to a cross-sectional view taken alongdashed-dotted line X5-X6 in FIG. 10A.

In the touch panel 2000 illustrated in FIG. 12A, the display device 2501described with reference to FIG. 11A and the touch sensor 2595 describedwith reference to FIG. 11C are attached to each other.

The touch panel 2000 illustrated in FIG. 12A includes an adhesive layer2597 and an anti-reflective layer 2567 p in addition to the componentsdescribed with reference to FIGS. 11A and 11C.

The adhesive layer 2597 is provided in contact with the wiring 2594.Note that the adhesive layer 2597 attaches the substrate 2590 to thesubstrate 2570 so that the touch sensor 2595 overlaps with the displaydevice 2501. The adhesive layer 2597 preferably has a light-transmittingproperty. A heat curable resin or an ultraviolet curable resin can beused for the adhesive layer 2597. For example, an acrylic resin, aurethane-based resin, an epoxy-based resin, or a siloxane-based resincan be used.

The anti-reflective layer 2567 p is positioned in a region overlappingwith pixels. As the anti-reflective layer 2567 p, a circularlypolarizing plate can be used, for example.

Next, a touch panel having a structure different from that illustratedin FIG. 12A will be described with reference to FIG. 12B.

FIG. 12B is a cross-sectional view of a touch panel 2001. The touchpanel 2001 illustrated in FIG. 12B differs from the touch panel 2000illustrated in FIG. 12A in the position of the touch sensor 2595relative to the display device 2501. Different parts are described indetail below, and the above description of the touch panel 2000 isreferred to for the other similar parts.

The coloring layer 2567R is positioned in a region overlapping with thelight-emitting element 2550R. The light-emitting element 2550Rillustrated in FIG. 12B emits light to the side where the transistor2502 t is provided. Accordingly, part of light emitted from thelight-emitting element 2550R passes through the coloring layer 2567R andis emitted to the outside of the light-emitting module 2580R asindicated by an arrow in FIG. 12B.

The touch sensor 2595 is provided on the substrate 2510 side of thedisplay device 2501.

The adhesive layer 2597 is provided between the substrate 2510 and thesubstrate 2590 and attaches the touch sensor 2595 to the display device2501.

As illustrated in FIG. 12A or 12B, light may be emitted from thelight-emitting element through one or both of the substrate 2510 and thesubstrate 2570.

<Description of Method for Driving Touch Panel>

Next, an example of a method for driving a touch panel will be describedwith reference to FIGS. 13A and 13B.

FIG. 13A is a block diagram illustrating the structure of a mutualcapacitive touch sensor. FIG. 13A illustrates a pulse voltage outputcircuit 2601 and a current sensing circuit 2602. Note that in FIG. 13A,six wirings X1 to X6 represent the electrodes 2621 to which a pulsevoltage is applied, and six wirings Y1 to Y6 represent the electrodes2622 that detect changes in current. FIG. 13A also illustratescapacitors 2603 that are each formed in a region where the electrodes2621 and 2622 overlap with each other. Note that functional replacementbetween the electrodes 2621 and 2622 is possible.

The pulse voltage output circuit 2601 is a circuit for sequentiallyapplying a pulse voltage to the wirings X1 to X6. By application of apulse voltage to the wirings X1 to X6, an electric field is generatedbetween the electrodes 2621 and 2622 of the capacitor 2603. When theelectric field between the electrodes is shielded, for example, a changeoccurs in the capacitor 2603 (mutual capacitance). The approach orcontact of a sensing target can be sensed by utilizing this change.

The current sensing circuit 2602 is a circuit for detecting changes incurrent flowing through the wirings Y1 to Y6 that are caused by thechange in mutual capacitance in the capacitor 2603. No change in currentvalue is sensed in the wirings Y1 to Y6 when there is no approach orcontact of a sensing target, whereas a decrease in current value issensed when mutual capacitance decreases due to the approach or contactof a sensing target. Note that an integrator circuit or the like is usedfor sensing of current values.

FIG. 13B is a timing chart showing input and output waveforms in themutual capacitive touch sensor illustrated in FIG. 13A. In FIG. 13B, asensing target is sensed in all the rows and columns in one frameperiod. FIG. 13B shows a period when a sensing target is not sensed (nottouched) and a period when a sensing target is sensed (touched). In FIG.13B, sensed current values of the wirings Y1 to Y6 are shown as thewaveforms of voltage values.

Pulse voltages are sequentially applied to the wirings X1 to X6, and thewaveforms of the wirings Y1 to Y6 change in accordance with the pulsevoltages. When there is no approach or contact of a sensing target, thewaveforms of the wirings Y1 to Y6 change uniformly in accordance withchanges in the voltages of the wirings X1 to X6. The current valuedecreases at the point of approach or contact of a sensing target andaccordingly the waveform of the voltage value changes.

By sensing a change in mutual capacitance in this manner, the approachor contact of a sensing target can be sensed.

<Description of Sensor Circuit>

The passive matrix type touch sensor in which only the capacitor 2603 isprovided at the intersection of wirings is illustrated as a touch sensorin FIG. 13A; alternatively, an active matrix type touch sensor includinga transistor and a capacitor may be used. FIG. 14 illustrates an exampleof a sensor circuit included in an active matrix type touch sensor.

The sensor circuit in FIG. 14 includes the capacitor 2603 andtransistors 2611, 2612, and 2613.

A signal G2 is input to a gate of the transistor 2613. A voltage VRES isapplied to one of a source and a drain of the transistor 2613, and oneelectrode of the capacitor 2603 and a gate of the transistor 2611 areelectrically connected to the other of the source and the drain of thetransistor 2613. One of a source and a drain of the transistor 2611 iselectrically connected to one of a source and a drain of the transistor2612, and a voltage VSS is applied to the other of the source and thedrain of the transistor 2611. A signal G1 is input to a gate of thetransistor 2612, and a wiring ML is electrically connected to the otherof the source and the drain of the transistor 2612. The voltage VSS isapplied to the other electrode of the capacitor 2603.

Next, the operation of the sensor circuit in FIG. 14 will be described.First, a potential for turning on the transistor 2613 is supplied as thesignal G2, and a potential corresponding to the voltage VRES is thusapplied to a node n connected to the gate of the transistor 2611. Then,a potential for turning off the transistor 2613 is applied as the signalG2, whereby the potential of the node n is maintained.

Then, mutual capacitance of the capacitor 2603 changes owing to theapproach or contact of a sensing target such as a finger, andaccordingly the potential of the node n is changed from VRES.

In reading operation, a potential for turning on the transistor 2612 issupplied as the signal G1. A current flowing through the transistor2611, that is, a current flowing through the wiring ML changes with thepotential of the node n. By sensing this current, the approach orcontact of a sensing target can be sensed.

In each of the transistors 2611, 2612, and 2613, an oxide semiconductorlayer is preferably used as a semiconductor layer in which a channelregion is formed. In particular, such a transistor is preferably used asthe transistor 2613 so that the potential of the node n can be held fora long time and the frequency of operation of resupplying VRES to thenode n (refresh operation) can be reduced.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 6

In this embodiment, a display module and electronic devices including alight-emitting element of one embodiment of the present invention willbe described with reference to FIG. 15 and FIGS. 16A to 16G.

<Display Module>

In a display module 8000 illustrated in FIG. 15, a touch sensor 8004connected to an FPC 8003, a display device 8006 connected to an FPC8005, a frame 8009, a printed board 8010, and a battery 8011 areprovided between an upper cover 8001 and a lower cover 8002.

The light-emitting element of one embodiment of the present inventioncan be used for the display device 8006, for example.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchsensor 8004 and the display device 8006.

The touch sensor 8004 can be a resistive touch sensor or a capacitivetouch sensor and may be formed to overlap with the display device 8006.A counter substrate (sealing substrate) of the display device 8006 canhave a touch sensor function. A photosensor may be provided in eachpixel of the display device 8006 so that an optical touch sensor isobtained.

The frame 8009 protects the display device 8006 and also serves as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may serve as aradiator plate.

The printed board 8010 includes a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or the battery 8011 provided separatelymay be used. The battery 8011 can be omitted in the case of using acommercial power source.

The display module 8000 can be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

<Electronic Devices>

FIGS. 16A to 16G illustrate electronic devices. These electronic devicescan include a housing 9000, a display portion 9001, a speaker 9003,operation keys 9005 (including a power switch or an operation switch), aconnection terminal 9006, a sensor 9007 (a sensor having a function ofmeasuring or sensing force, displacement, position, speed, acceleration,angular velocity, rotational frequency, distance, light, liquid,magnetism, temperature, chemical substance, sound, time, hardness,electric field, current, voltage, electric power, radiation, flow rate,humidity, gradient, oscillation, odor, or infrared ray), a microphone9008, and the like.

The electronic devices illustrated in FIGS. 16A to 16G can have avariety of functions, for example, a function of displaying a variety ofdata (a still image, a moving image, a text image, and the like) on thedisplay portion, a touch sensor function, a function of displaying acalendar, date, time, and the like, a function of controlling a processwith a variety of software (programs), a wireless communicationfunction, a function of being connected to a variety of computernetworks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, and a function of reading a program or datastored in a memory medium and displaying the program or data on thedisplay portion. Note that the functions of the electronic devicesillustrated in FIGS. 16A to 16G are not limited to those describedabove, and the electronic devices can have a variety of functions.Although not illustrated in FIGS. 16A to 16G, the electronic devices mayinclude a plurality of display portions. The electronic devices mayinclude a camera or the like and have a function of taking a stillimage, a function of taking a moving image, a function of storing thetaken image in a memory medium (an external memory medium or a memorymedium incorporated in the camera), a function of displaying the takenimage on the display portion, or the like.

The electronic devices illustrated in FIGS. 16A to 16G will be describedin detail below.

FIG. 16A is a perspective view of a portable information terminal 9100.The display portion 9001 of the portable information terminal 9100 isflexible. Therefore, the display portion 9001 can be incorporated alonga bent surface of a bent housing 9000. In addition, the display portion9001 includes a touch sensor, and operation can be performed by touchingthe screen with a finger, a stylus, or the like. For example, when anicon displayed on the display portion 9001 is touched, an applicationcan be started.

FIG. 16B is a perspective view of a portable information terminal 9101.The portable information terminal 9101 functions as, for example, one ormore of a telephone set, a notebook, and an information browsing system.Specifically, the portable information terminal can be used as asmartphone. Note that the speaker 9003, the connection terminal 9006,the sensor 9007, and the like, which are not illustrated in FIG. 16B,can be positioned in the portable information terminal 9101 as in theportable information terminal 9100 illustrated in FIG. 16A. The portableinformation terminal 9101 can display characters and image informationon its plurality of surfaces. For example, three operation buttons 9050(also referred to as operation icons, or simply, icons) can be displayedon one surface of the display portion 9001. Furthermore, information9051 indicated by dashed rectangles can be displayed on another surfaceof the display portion 9001. Examples of the information 9051 includedisplay indicating reception of an incoming email, social networkingservice (SNS) message, call, and the like; the title and sender of anemail and SNS message; the date; the time; remaining battery level; andthe reception strength of an antenna. Alternatively, the operationbuttons 9050 or the like may be displayed in place of the information9051.

FIG. 16C is a perspective view of a portable information terminal 9102.The portable information terminal 9102 has a function of displayinginformation on three or more surfaces of the display portion 9001. Here,information 9052, information 9053, and information 9054 are displayedon different surfaces. For example, a user of the portable informationterminal 9102 can see the display (here, the information 9053) with theportable information terminal 9102 put in a breast pocket of his/herclothes. Specifically, a caller's phone number, name, or the like of anincoming call is displayed in a position that can be seen from above theportable information terminal 9102. Thus, the user can see the displaywithout taking out the portable information terminal 9102 from thepocket and decide whether to answer the call.

FIG. 16D is a perspective view of a watch-type portable informationterminal 9200. The portable information terminal 9200 is capable ofexecuting a variety of applications such as mobile phone calls,e-mailing, viewing and editing texts, music reproduction, Internetcommunication, and computer games. The display surface of the displayportion 9001 is bent, and images can be displayed on the bent displaysurface. The portable information terminal 9200 can employ near fieldcommunication that is a communication method based on an existingcommunication standard. In that case, for example, mutual communicationbetween the portable information terminal 9200 and a headset capable ofwireless communication can be performed, and thus hands-free calling ispossible. The portable information terminal 9200 includes the connectionterminal 9006, and data can be directly transmitted to and received fromanother information terminal via a connector. Power charging through theconnection terminal 9006 is also possible. Note that the chargingoperation may be performed by wireless power feeding without using theconnection terminal 9006.

FIGS. 16E, 16F, and 16G are perspective views of a foldable portableinformation terminal 9201. FIG. 16E is a perspective view illustratingthe portable information terminal 9201 that is opened. FIG. 16F is aperspective view illustrating the portable information terminal 9201that is being opened or being folded. FIG. 16G is a perspective viewillustrating the portable information terminal 9201 that is folded. Theportable information terminal 9201 is highly portable when folded. Whenthe portable information terminal 9201 is opened, a seamless largedisplay region is highly browsable. The display portion 9001 of theportable information terminal 9201 is supported by three housings 9000joined together by hinges 9055. By folding the portable informationterminal 9201 at a connection portion between two housings 9000 with thehinges 9055, the portable information terminal 9201 can be reversiblychanged in shape from an opened state to a folded state. For example,the portable information terminal 9201 can be bent with a radius ofcurvature greater than or equal to 1 mm and less than or equal to 150mm.

The electronic devices in this embodiment each include a display portionfor displaying some kind of information. The light-emitting element ofone embodiment of the present invention can also be used for electronicdevices without a display portion. In the electronic devices describedin this embodiment, the display portion is flexible and display can beperformed on the curved display surface, or the display portion isfoldable; however, the structure of the electronic devices is notlimited to these examples, and a structure in which the display portionis not flexible and display is performed on a plane portion may beemployed.

The structure described in this embodiment can be combined with any ofthe structures described in the other embodiments as appropriate.

Embodiment 7

In this embodiment, examples of lighting devices using thelight-emitting element of one embodiment of the present invention willbe described with reference to FIG. 17.

FIG. 17 illustrates an example in which the light-emitting element isused for an indoor lighting device 8501. Since the light-emittingelement can have a larger area, a lighting device having a large areacan also be formed. In addition, a lighting device 8502 in which alight-emitting region has a curved surface can also be formed with theuse of a housing with a curved surface. The light-emitting elementdescribed in this embodiment is in the form of a thin film, which allowsthe housing to be designed more freely. Therefore, the lighting devicecan be elaborately designed in a variety of ways. Furthermore, a wall ofthe room may be provided with a large-sized lighting device 8503. Touchsensors may be provided in the lighting devices 8501, 8502, and 8503 tocontrol the power on/off of the lighting devices.

Moreover, when the light-emitting element is used on the surface side ofa table, a lighting device 8504 which has a function as a table can beobtained. When the light-emitting element is used as part of otherfurniture, a lighting device which has a function as the furniture canbe obtained.

In this manner, a variety of lighting devices using the light-emittingelement can be obtained. Note that such lighting devices are included inone embodiment of the present invention.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Example 1

In this example, eight light-emitting elements with different structureswere fabricated. Note that the eight light-emitting elements includefour light-emitting elements with different materials used forelectron-transport layers, and four light-emitting elements differentfrom the former four light-emitting elements only in a material used forhole-transport layers. Note that the fabrication of the light-emittingelements 1 to 8 is described with reference to FIG. 18. The chemicalformulae of materials used in this example are as follows.

<<Fabrication of Light-Emitting Elements 1 to 8>>

First, indium tin oxide (ITO) containing silicon oxide was depositedover a glass substrate 900 by a sputtering method, whereby a firstelectrode 901 functioning as an anode was formed. Note that the firstelectrode has a thickness of 70 nm and an area of 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 900, a surface of the substrate was washed with water andbaking was performed at 200° C. for 1 hour; then, UV ozone treatment wasperformed for 370 seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 1×10⁻⁴Pa, and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus. Then, the substrate900 was cooled down for approximately 30 minutes.

Next, the substrate 900 was fixed to a holder provided in the vacuumevaporation apparatus so that a surface of the substrate on which thefirst electrode 901 was formed faced downward. In this example, ahole-injection layer 911, a hole-transport layer 912, a light-emittinglayer 913, an electron-transport layer 914, and an electron-injectionlayer 915, which are included in an EL layer 902, are sequentiallyformed by a vacuum evaporation method.

After the pressure in the vacuum evaporation apparatus was reduced to1×10⁻⁴ Pa, in the case of the light-emitting elements 1 to 4,3-[4-(9-phenanthryl)-phenyl]-9-phenyl-9H-carbazole (abbreviation: PCPPn)and molybdenum oxide were deposited by co-evaporation with a weightratio of PCPPn to molybdenum oxide being 4:2, so that the hole-injectionlayer 911 was formed on the first electrode 901. The thickness was setto 10 nm. In the case of the light-emitting elements 5 to 8,9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA) and molybdenum oxide were deposited by co-evaporation with aweight ratio of PCzPA to molybdenum oxide being 4:2, so that thehole-injection layer 911 was formed on the first electrode 901. Thethickness was set to 10 nm. Note that the co-evaporation is anevaporation method in which a plurality of different substances areconcurrently vaporized from respective different evaporation sources.

Next, in the case of the light-emitting elements 1 to 4, PCPPn wasdeposited by evaporation to a thickness of 30 nm on the hole-injectionlayer 911, so that the hole-transport layer 912 was formed. In the caseof the light-emitting elements 5 to 8, PCzPA was deposited byevaporation to a thickness of 30 nm on the hole-injection layer 911, sothat the hole-transport layer 912 was formed.

Then, on the hole-transport layer 912,7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(abbreviation: cgDBCzPA), andN,N-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(abbreviation: 1,6mMemFLPAPrn) were deposited by co-evaporation with aweight ratio of cgDBCzPA to 1,6mMemFLPAPrn being 1:0.03, so that thelight-emitting layer 913 was formed. Note that the thickness was set to25 nm.

Next, in the case of the light-emitting elements 1 and 5,bathophenanthroline (abbreviation: BPhen) was deposited by evaporationto a thickness of 25 nm on the light-emitting layer 913, so that theelectron-transport layer 914 was formed. In the case of thelight-emitting elements 2 and 6,2,2′-(pyridine-2,6-diyl)bis(4,6-diphenylpyrimidine) (abbreviation:2,6(P2Pm)2Py) was deposited by evaporation to a thickness of 25 nm onthe light-emitting layer 913, so that the electron-transport layer 914was formed. In the case of the light-emitting elements 3 and 7,2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline (abbreviation:NBPhen) was deposited by evaporation to a thickness of 25 nm on thelight-emitting layer 913, so that the electron-transport layer 914 wasformed. In the case of the light-emitting elements 4 and 8,2,2′-(pyridine-2,6-diyl)bis(4-phenylbenzo[h]quinazoline) (abbreviation:2,6(P-Bqn)2Py) was deposited by evaporation to a thickness of 25 nm onthe light-emitting layer 913, so that the electron-transport layer 914was formed.

Furthermore, lithium fluoride was deposited by evaporation to athickness of 1 nm on the electron-transport layer 914, so that theelectron-injection layer 915 was formed.

Finally, aluminum was deposited by evaporation to a thickness of 200 nmon the electron-injection layer 915, so that a second electrode 903functioning as a cathode was formed. Thus, the light-emitting elements 1to 8 were fabricated. Note that in all the above evaporation steps,evaporation was performed by a resistance-heating method.

Table 1 shows the structures of the thus obtained light-emittingelements 1 to 8.

TABLE 1 Hole- Light- First Hole-injection transport emitting Electron-Electron- Second electrode layer layer layer transport layer injectionlayer electrode Light-emitting ITO PCPPn: PCPPn * Bphen LiF Al element 1(70 nm) MoOx (30 nm) (25 nm) (1 nm) (200 nm) (4:2 10 nm) Light-emittingITO PCPPn: PCPPn * 2,6(P2Pm)2Py LiF Al element 2 (70 nm) MoOx (30 nm)(25 nm) (1 nm) (200 nm) (4:2 10 nm) Light-emitting ITO PCPPn: PCPPn *NBPhen LiF Al element 3 (70 nm) MoOx (30 nm) (25 nm) (1 nm) (200 nm)(4:2 10 nm) Light-emitting ITO PCPPn: PCPPn 2,6(P-Bqn)2Py LiF Al element4 (70 nm) MoOx (30 nm) * (25 nm) (1 nm) (200 nm) (4:2 10 nm)Light-emitting ITO PCzPA: PCzPA * Bphen LF Al element 5 (70 nm) MoOx (30nm) (25 nm) (1 nm) (200 nm) (4:2 10 nm) Light-emitting ITO PCzPA:PCzPA * 2,6(P2Pm)2Py LiF Al element 6 (70 nm) MoOx (30 nm) (25 nm) (1nm) (200 nm) (4:2 10 nm) Light-emitting ITO PCzPA: PCzPA * NBPhen LiF Alelement 7 (70 nm) MoOx (30 nm) (25 nm) (1 nm) (200 nm) (4:2 10 nm)Light-emitting ITO PCzPA: PCzPA * 2,6(P-Bqn)2Py LiF Al element 8 (70 nm)MoOx (30 nm) (25 nm) (1 nm) (200 nm) (4:2 10 nm) *cgDBCzPA:1,6mMemFLPAPm(10.03 25 nm)

The fabricated light-emitting elements 1 to 8 were sealed in a glove boxunder a nitrogen atmosphere so as not to be exposed to the air (asealant was applied to surround the elements, and at the time ofsealing, UV treatment was performed and heat treatment was performed at80° C. for 1 hour). Note that the number of each light-emitting elementfabricated for the measurements below is four.

<<Properties of Light-Emitting Elements 1 to 8>>

The fabricated light-emitting elements 1 to 8 (the number of eachelement is four) were measured using a picosecond fluorescence lifetimemeasurement system (manufactured by Hamamatsu Photonics K.K.). Tomeasure the lifetimes of fluorescence in the light-emitting elements, asquare wave pulse voltage was applied to the light-emitting elements,and time-resolved measurements of light, which was attenuated from thefalling of the voltage, were performed using a streak camera. The pulsevoltage was applied at a frequency of 10 Hz. By integrating dataobtained by repeated measurements, data with a high S/N ratio wasobtained. The measurement was performed at room temperature (300 K)under the conditions of a pulse voltage of approximately 3 V, a pulsetime width of 100 μsec, a negative bias voltage of −5 V, and ameasurement time of 50 μsec.

The attenuation curve of transient fluorescence obtained by themeasurement was fitted with Formula (f1).

$\begin{matrix}{L = {\sum\limits_{n = 1}{A_{n}{\exp( {- \frac{t}{a_{n}}} )}}}} & ({f1})\end{matrix}$

Note that in Formula (f1), L and t represent normalized emissionintensity and elapsed time, respectively.

The fitting of the attenuation curve obtained by the measurement wasable to be performed with n=1 and 2. It was also found that thelight-emitting elements 1 to 8 each included a delayed fluorescencecomponent as well as a fluorescence component. Note that the delayedfluorescence component refers to fluorescence that is measuredimmediately after the pulse voltage is shut off, i.e., after theinjection of carriers to a light-emitting layer is stopped. The measureddelayed fluorescence component means the generation of triplet-tripletannihilation (TTA) in the EL layer of the light-emitting element. Theproportion of the delayed fluorescence component refers to theproportion of the fluorescence intensity of a light-emitting elementimmediately after the pulse voltage is shut off to the fluorescenceintensity thereof when the pulse voltage is supplied; in other words,the proportion of the fluorescence intensity of the light-emittingelement immediately after carrier injection is stopped to thefluorescence intensity thereof when carriers are steadily injected to alight-emitting layer. Table 2 below shows the proportion of the delayedfluorescence component of each of the light-emitting elements 1 to 8.

In addition, the emission properties of the light-emitting elements 1 to8 were measured to obtain the external quantum efficiency. Note that theexternal quantum efficiency of each light-emitting element was obtainedin such a manner that the viewing angle dependence of light was measuredwhile the substrate was rotated at an angle of −80° to 80° and the lightdistribution characteristics of electroluminescence were taken intoconsideration. The results are shown in Table 2 below. Note that themeasurement was performed at room temperature (in an atmospheremaintained at 25° C.).

TABLE 2 Proportion of delayed External fluorescence quantum Pulsevoltage component (%) efficiency (%) (V) Light-emitting 12.3 8.97 3.15element 1 12.5 9.04 3.10 12.1 9.17 3.10 10.9 9.02 3.10 Light-emitting13.7 9.12 3.10 element 2 12.0 9.21 3.10 13.0 9.17 3.10 14.0 9.23 3.10Light-emitting 18.1 9.37 3.15 element 3 16.2 9.79 3.15 17.4 9.76 3.1518.2 9.72 3.15 Light-emitting 20.7 9.66 3.20 element 4 20.4 9.76 3.2019.5 9.55 3.20 20.9 9.33 3.20 Light-emitting 8.8 7.71 3.00 element 5 7.77.83 3.00 6.8 7.91 3.05 6.6 7.84 2.95 Light-emitting 7.3 7.83 3.00element 6 7.1 7.92 3.00 8.7 8.02 3.05 7.7 7.96 3.00 Light-emitting 10.17.91 3.00 element 7 10.2 8.29 3.05 9.4 8.34 3.05 11.1 8.23 3.05Light-emitting 12.5 8.09 3.20 element 8 12.0 8.13 3.15 12.4 8.09 3.2013.4 7.87 3.20

Table 3 shows the LUMO levels of the materials used in theelectron-transport layers of the light-emitting elements 1 to 8. TheLUMO levels were estimated from the cyclic voltammetry measurements ofeach material in a N,N-dimethylformamide (abbreviation: DMF) solvent.

TABLE 3 Electron-transport LUMO level layer (eV) Light-emitting elementsBphen −2.63 1 and 5 Light-emitting elements 2,6(P2Pm)2Py −2.78 2 and 6Light-emitting elements NBPhen −2.83 3 and 7 Light-emitting elements2,6(P-Bqn)2Py −2.92 4 and 8

FIG. 19 shows the proportion of the delayed fluorescence component (%)versus the LUMO level (eV) (shown in Table 3) of the materials used inthe electron-transport layers of the light-emitting elements 1 to 8. InFIG. 19, the light-emitting elements 1 to 4, which use PCPPn in thehole-transport layers, are commonly plotted and the light-emittingelements 5 to 8, which use PCzPA in the hole-transport layers, arecommonly plotted.

The results indicate that the proportion of the delayed fluorescencecomponent tends to increase as the LUMO level of the material used inthe electron-transport layer becomes higher, though the proportion has adifferent value for each material used in the hole-transport layer. Thatis, the light-emitting elements using 2,6(P-Bqn)2Py, which has thehighest LUMO level (−2.92 eV), have a higher proportion of delayedfluorescence component than the other light-emitting elements having thesame structure except for the electron-transport layer.

FIG. 20 shows the relationship between the proportion of the delayedfluorescence component (%) and the external quantum efficiency (%) ofthe light-emitting elements 1 to 8.

The results show that the external quantum efficiency (%) increases withthe proportion of delayed fluorescence component (%).

Here, the proportion of delayed fluorescence component (X) isrepresented by Formula (f2) where I_(P) is the fluorescence intensity inthe direct formation process and I_(D) is the intensity of delayedfluorescence due to TTA.

$\begin{matrix}{X = \frac{I_{D}}{I_{P} + I_{D}}} & ({f2})\end{matrix}$

According to the definition of the external quantum efficiency (EQE),the proportion of generated singlet excitons (α) is 0.25 when I_(D) is 0(X=0). However, α increases with I_(D), i.e., EQE is proportional toI_(P)+I_(D). Accordingly, Formula (f3) is obtained.

$\begin{matrix}{{{EQE} \propto {I_{P} + I_{D}}} = {I_{P} \times \frac{1}{1 - X}}} & ({f3})\end{matrix}$

Since X is equivalent to the x-axis in FIG. 20, the relationship betweenthe proportion of the delayed fluorescence component (%) and theexternal quantum efficiency (%) of the light-emitting elements 1 to 4,which use PCPPn in the hole-transport layers, and the relationshipbetween the proportion of the delayed fluorescence component (%) and theexternal quantum efficiency (%) of the light-emitting elements 5 to 8,which use PCzPA in the hole-transport layers, can be substituted intoFormula (f3) as shown in FIG. 20. This shows that the proportion of thedelayed fluorescence component (%) and the external quantum efficiency(%) have a correlation.

Furthermore, a light-emitting element 4-2, which has the same structureas the light-emitting element 4 shown in Table 1, was fabricated and thecharacteristics thereof were measured. Note that FIG. 21 shows thecurrent density-luminance characteristics of the light-emitting element4-2; FIG. 22, the voltage-luminance characteristics thereof; FIG. 23,the luminance-current efficiency characteristics thereof; FIG. 24, thevoltage-current characteristics thereof; and FIG. 25, theluminance-external quantum efficiency characteristics thereof. Here, theexternal quantum efficiency of the light-emitting element shown in FIG.25 was obtained in the aforementioned manner that the viewing angledependence of light was measured while the substrate was rotated at anangle of −80° to 80° and the light distribution characteristics ofelectroluminescence were taken into consideration. The characteristicvalues of the light-emitting elements shown in FIGS. 21 to 24 werecalculated from front luminance with a luminance colorimeter BM-5A(manufactured by TOPCON TECHNOHOUSE CORPORATION).

Table 4 shows the initial values of main characteristics of thelight-emitting element 4-2 at a luminance of approximately 1000 cd/m².

TABLE 4 External Current Current Power quantum Voltage densityChromaticity Luminance efficiency efficiency efficiency (V) (mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Light-emitting 3.2 8.6 (0.14, 0.14) 1100 1313 9.0 element 4-2

FIG. 26 shows an emission spectrum when a current with a current densityof 12.5 mA/cm² was supplied to the light-emitting element 4-2. As shownin FIG. 26, the emission spectrum of the light-emitting element 4-2 hasa peak at around 464 nm, which is probably derived from 1,6mMemFLPAPrnused as the guest material (dopant) in the light-emitting layer of thelight-emitting element 4-2.

FIG. 27 shows an attenuation curve of transient fluorescence of thelight-emitting element 4-2. Note that in FIG. 27, the vertical axisrepresents the emission intensity normalized to that in a state wherecarriers are steadily injected (when the pulse voltage is supplied), andthe horizontal axis represents the time elapsed after the falling of thepulse voltage. The attenuation curve shown in FIG. 27 was fitted withFormula (f1), whereby a proportion of delayed fluorescence component of21.5% was obtained.

Next, the light-emitting element 4-2 was subjected to a reliabilitytest. FIG. 28 shows the results of the reliability test. In FIG. 28, thevertical axis represents normalized luminance (%) with an initialluminance of 100% and the horizontal axis represents driving time (h) ofthe element. Note that in the reliability test, the light-emittingelement 4-2 was driven under the conditions where the initial luminancewas set to 5000 cd/m² and the current density was constant.

As a result, the light-emitting element 4-2 of one embodiment of thepresent invention showed a high reliability.

Example 2

In this example, the transition dipole moment orientation of a moleculecontributing to light emission in a light-emitting layer of alight-emitting element was estimated. Specifically, the angulardependence of the spectrum intensity of a p-polarized emission componentwas measured and the results were analyzed by calculation (simulation),whereby the transition dipole moment orientation of a molecule wasestimated. Among the materials used in this example, materials that arenot described in Example 1 are represented by the following chemicalformulae.

<<Fabrication of Light-Emitting Element 9>>

First, the fabrication of a light-emitting element 9 to be measured willbe described with reference to FIG. 18. First, indium tin oxide (ITO)containing silicon oxide was deposited over the glass substrate 900 by asputtering method, whereby the first electrode 901 functioning as ananode was formed. Note that the first electrode has a thickness of 70 nmand an area of 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 900, a surface of the substrate was washed with water andbaking was performed at 200° C. for 1 hour; then, UV ozone treatment wasperformed for 370 seconds.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 1×10⁻⁴Pa, and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus. Then, the substrate900 was cooled down for approximately 30 minutes.

Next, the substrate 900 was fixed to a holder provided in the vacuumevaporation apparatus so that a surface of the substrate on which thefirst electrode 901 was formed faced downward. In this example, thehole-injection layer 911, the hole-transport layer 912, thelight-emitting layer 913, the electron-transport layer 914, and theelectron-injection layer 915, which are included in the EL layer 902,are sequentially formed by a vacuum evaporation method.

After the pressure in the vacuum evaporation apparatus was reduced to1×10⁻⁴ Pa, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation:DBT3P-II) and molybdenum oxide were deposited by co-evaporation with aweight ratio of DBT3P-II to molybdenum oxide being 2:1, so that thehole-injection layer 911 was formed on the first electrode 901. Thethickness was set to 10 nm. Note that the co-evaporation is anevaporation method in which a plurality of different substances areconcurrently vaporized from respective different evaporation sources.

Then, BPAFLP was deposited by evaporation to a thickness of 30 nm on thehole-injection layer 911, so that the hole-transport layer 912 wasformed.

Then, on the hole-transport layer 912,7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole (cgDBCzPA),andN,N′-bis(3-methylphenyl)-N,N′-bis[3-(9-phenyl-9H-fluoren-9-yl)phenyl]pyrene-1,6-diamine(1,6mMemFLPAPrn) were deposited by co-evaporation with a weight ratio ofcgDBCzPA to 1,6mMemFLPAPrn being 1:0.03, so that the light-emittinglayer 913 was formed. Note that the thickness was set to 15 nm.

Next, 7-[4-(10-phenyl-9-anthryl)phenyl]-7H-dibenzo[c,g]carbazole(cgDBCzPA) was formed to a thickness of 20 nm on the light-emittinglayer 913, and 2,9-bis(naphthalen-2-yl)-4,7-diphenyl-1,10-phenanthroline(abbreviation: NBPhen) was deposited by evaporation to a thickness of 15nm thereon, so that the electron-transport layer 914 was formed.

Furthermore, lithium oxide was deposited by evaporation to a thicknessof 0.1 nm on the electron-transport layer 914, copper phthalocyanine(CuPc) was then deposited by evaporation to a thickness of 2 nm, and1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) andmolybdenum oxide were deposited by co-evaporation to a thickness of 60nm with a weight ratio of DBT3P-II to molybdenum oxide being 2:1, sothat the electron-injection layer 915 was formed.

Finally, aluminum was deposited by evaporation to a thickness of 200 nmon the electron-injection layer 915, so that the second electrode 903functioning as a cathode was formed. Thus, the light-emitting element 9was fabricated. Note that in all the above evaporation steps,evaporation was performed by a resistance-heating method.

The thickness of each layer in the light-emitting element 9 wasdetermined to obtain the minimum front luminance. This relativelyincreases the luminance derived from a transition dipole moment having acomponent in a direction perpendicular to the light-emitting layer 913,and facilitates the measurement of the light emission.

<<Polarization Measurement>>

Measurements will be described next. A multi-channel spectrometer(PMA-12 produced by Hamamatsu Photonics K.K.) was used as a detector. Apolarizer produced by Edmund Optics Inc. was placed on the optical pathfrom the light-emitting element 9 to the detector, so that only acomponent parallel to the observation direction reached the detector.The light that had passed through the polarizer was detected in PMA-12(multi-channel spectrometer produced by Hamamatsu Photonics K.K.) and anemission spectrum was obtained. At this time, the substrate was rotatedangle by angle from 0° to 80° with the front of the emission surface ofthe substrate set to 0°, the emission spectrum at each angle wasmeasured, and the integrated intensity of the spectrum was plotted.

<<Calculation (Simulation)>>

Calculation will be described next. The calculation was performed usingan organic device simulator “setfos” developed by Cybernet SystemsCorporation. Fitting was performed while the stacked structure of anelement, the film thickness, the refractive index n and extinctioncoefficient k of each layer, the emission position, and the emissionspectrum were set as parameters and the degree of transition dipolemoment orientation of a light-emitting molecule was set as a variableparameter (parameter a described later). Note that the emission positionwas assumed to be in the vicinity of the interface between thehole-transport layer and the light-emitting layer. The thickness of eachlayer was obtained from the value of a quartz oscillator (rate monitor)in an evaporation apparatus at the time of fabricating the samples, andthe refractive index n and the extinction coefficient k were obtainedfrom the analysis results of spectroscopic ellipsometry. The emissionspectrum was obtained by photoluminescence (PL) measurement of a thinfilm.

The degree of transition dipole moment orientation is defined as theparameter a. That is, the parameter a indicates the proportion of acomponent perpendicular to a light-emitting layer in the transitiondipole moment in the total components perpendicular and parallel to thelight-emitting layer. The transition dipole moment includes only thecomponent perpendicular to the light-emitting layer when a=1, whereas itincludes only the component parallel to the light-emitting layer whena=0. The transition dipole moment of molecules isotropically orientedhas the same component in the x direction, the y direction, and the zdirection which are orthogonal to one another; in that case, a=0.33.

<<Fitting of Measured Results and Calculation (Simulation)>>

FIG. 29 shows the plotted values of measured angular dependence and thecalculation results where a=0 (the transition dipole moment iscompletely horizontal), a=0.16, a=0.33 (the transition dipole moment israndomly oriented), and a=1 (the transition dipole moment is completelyvertical). The calculation results where 84% of the transition dipolemoment component is a component in a direction parallel to thelight-emitting layer and 16% thereof is a component in a verticaldirection (a=0.16) were almost equivalent to the plotted values ofmeasured angular dependence. This leads to the assumption that 84% ofthe transition dipole moment component of emission molecules included inthe light-emitting layer 913 of the light-emitting element 9 is in adirection parallel to the light-emitting layer 913, i.e., most of thetransition dipole moment is deviated from the direction perpendicular tothe light-emitting layer.

The above results show that the emission molecules in the light-emittinglayer are oriented not randomly but strongly. These strongly orientedemission molecules probably cause a relatively high emission efficiencyof the light-emitting element of one embodiment of the presentinvention. Hence, in the light-emitting element of one embodiment of thepresent invention, in the case where the transition dipole moment of thelight-emitting material (the guest material in this example) is dividedinto a component parallel to the light-emitting layer and a componentperpendicular to the light-emitting layer, the proportion of thecomponent parallel to the light-emitting layer is preferably higher thanor equal to 80% and lower than or equal to 100%.

This application is based on Japanese Patent Application serial No.2015-234485 filed with Japan Patent Office on Dec. 1, 2015, and JapanesePatent Application serial No. 2016-051071 filed with Japan Patent Officeon Mar. 15, 2016, the entire contents of which are hereby incorporatedby reference.

EXPLANATION OF REFERENCE

-   100: EL layer 101: electrode 101 a: conductive layer 101 b:    conductive layer 102: electrode 103: electrode 103 a: conductive    layer 103 b: conductive layer 104: electrode 104 a: conductive layer    104 b: conductive layer 111: hole-injection layer 112:    hole-transport layer 113: electron-transport layer 114:    electron-injection layer 115: charge-generation layer 116:    hole-injection layer 117: hole-transport layer 118:    electron-transport layer 119: electron-injection layer 123B:    light-emitting layer 123G: light-emitting layer 123R: light-emitting    layer 130: light-emitting layer 131: host material 132: guest    material 140: partition wall 150: light-emitting element 160:    light-emitting layer 170: light-emitting layer 170 b: light-emitting    layer 180: observation direction of detector 181: transition dipole    moment component 182: transition dipole moment component 183:    transition dipole moment component 185: detector 200: substrate 220:    substrate 221B: region 221G: region 221R: region 222B: region 222G:    region 222R: region 223: light-blocking layer 224B: optical element    224G: optical element 224R: optical element 250: light-emitting    element 252: light-emitting element 254: light-emitting element 400:    EL layer 401: electrode 402: electrode 411: hole-injection layer    412: hole-transport layer 413: electron-transport layer 414:    electron-injection layer 416: hole-injection layer 417:    hole-transport layer 418: electron-transport layer 419:    electron-injection layer 420: light-emitting layer 421: host    material 422: guest material 430: light-emitting layer 431: host    material 431_1: organic compound 431_2: organic compound 432: guest    material 441: light-emitting unit 442: light-emitting unit 445:    charge-generation layer 450: light-emitting element 452:    light-emitting element 801: pixel circuit 802: pixel portion 804:    driver circuit portion 804 a: scan line driver circuit 804 b: signal    line driver circuit 806: protection circuit 807: terminal portion    852: transistor 854: transistor 862: capacitor 872: light-emitting    element 900: substrate 901: first electrode 902: EL layer 903:    second electrode 911: hole-injection layer 912: hole-transport layer    913: light-emitting layer 914: electron-transport layer 915:    electron-injection layer 2000: touch panel 2001: touch panel 2501:    display device 2502R: pixel 2502 t: transistor 2503 c: capacitor    2503 g: scan line driver circuit 2503 t: transistor 2509: FPC 2510:    substrate 2510 a: insulating layer 2510 b: flexible substrate 2510    c: adhesive layer 2511: wiring 2519: terminal 2521: insulating layer    2528: partition wall 2550R: light-emitting element 2560: sealing    layer 2567BM: light-blocking layer 2567 p: anti-reflective layer    2567R: coloring layer 2570: substrate 2570 a: insulating layer 2570    b: flexible substrate 2570 c: adhesive layer 2580R: light-emitting    module 2590: substrate 2591: electrode 2592: electrode 2593:    insulating layer 2594: wiring 2595: touch sensor 2597: adhesive    layer 2598: wiring 2599: adhesive layer 2601: pulse voltage output    circuit 2602: current sensing circuit 2603: capacitor 2611:    transistor 2612: transistor 2613: transistor 2621: electrode 2622:    electrode 8000: display module 8001: upper cover 8002: lower cover    8003: FPC 8004: touch sensor 8005: FPC 8006: display device 8009:    frame 8010: printed board 8011: battery 8501: lighting device 8502:    lighting device 8503: lighting device 8504: lighting device 9000:    housing 9001: display portion 9003: speaker 9005: operation key    9006: connection terminal 9007: sensor 9008: microphone 9050:    operation button 9051: information 9052: information 9053:    information 9054: information 9055: hinge 9100: portable information    terminal 9101: portable information terminal 9102: portable    information terminal 9200: portable information terminal 9201:    portable information terminal

1. (canceled)
 2. A light-emitting element comprising: an anode; acathode; and an EL layer between the anode and the cathode, wherein theEL layer includes a light-emitting layer and an electron-transport layerin contact with the light-emitting layer, wherein the light-emittinglayer includes a host material, wherein the electron-transport layerincludes a first material, wherein a LUMO level of the first material islower than a LUMO level of the host material, and wherein a differencebetween an energy of a peak wavelength of a fluorescence of the hostmaterial and an energy of a peak wavelength of a phosphorescence of thehost material is greater than 0.2 eV.
 3. The light-emitting elementaccording to claim 2, wherein the difference between the energy of thepeak wavelength of the fluorescence of the host material and the energyof the peak wavelength of the phosphorescence of the host material isgreater than or equal to 0.5 eV.
 4. The light-emitting element accordingto claim 2, wherein the first material is a substance including apyrazine skeleton or a pyrimidine skeleton.
 5. The light-emittingelement according to claim 2, further comprising a hole-transport layerin contact with the light-emitting layer, wherein the hole-transportlayer includes a second material, and wherein a LUMO level of the secondmaterial is higher than the LUMO level of the host material.
 6. Thelight-emitting element according to claim 2, further comprising ahole-transport layer in contact with the light-emitting layer, whereinthe hole-transport layer includes a second material, and wherein atriplet excitation energy of the second material is higher than thetriplet excitation energy of the substance that has the highest tripletexcitation energy among the materials contained in the light-emittinglayer by greater than or equal to 0.2 eV.
 7. The light-emitting elementaccording to claim 2, further comprising a fluorescent material.
 8. Thelight-emitting element according to claim 7, wherein a tripletexcitation energy of the fluorescent material is higher than a tripletexcitation energy of the host material.
 9. The light-emitting elementaccording to claim 7, wherein a LUMO level of the fluorescent materialis higher than or equal to the LUMO level of the host material.
 10. Thelight-emitting element according to claim 2, wherein the light-emittinglayer emits blue light.
 11. The light-emitting element according toclaim 2, wherein the LUMO level of the first material is lower than theLUMO level of the host material by greater than or equal to 0.05 eV. 12.The light-emitting element according to claim 2, wherein the firstmaterial is a substance including a diazine skeleton or a triazineskeleton.
 13. A light-emitting element comprising: an anode; a cathode;and an EL layer between the anode and the cathode, wherein the EL layerincludes a light-emitting layer, an electron-transport layer in contactwith the light-emitting layer, and a hole-injection layer, wherein thelight-emitting layer includes a host material, wherein theelectron-transport layer includes a first material, wherein thehole-injection layer comprises a substance including a halogen group,wherein a LUMO level of the first material is lower than a LUMO level ofthe host material, and wherein a difference between a triplet excitationenergy of the host material and a singlet excitation energy of the hostmaterial is greater than 0.2 eV.
 14. The light-emitting elementaccording to claim 13, wherein the difference between the tripletexcitation energy of the host material and the singlet excitation energyof the host material is greater than or equal to 0.5 eV.
 15. Thelight-emitting element according to claim 13, wherein the first materialis a substance including a diazine skeleton or a triazine skeleton. 16.The light-emitting element according to claim 13, wherein the firstmaterial is a substance including a pyrazine skeleton or a pyrimidineskeleton.
 17. A light-emitting element comprising: an anode; a cathode;and an EL layer between the anode and the cathode, wherein the EL layerincludes a light-emitting layer, an electron-transport layer in contactwith the light-emitting layer, and a hole-injection layer, wherein thelight-emitting layer includes a host material, wherein theelectron-transport layer includes a first material, wherein thehole-injection layer comprises a substance including a cyano group,wherein a LUMO level of the first material is lower than a LUMO level ofthe host material, and wherein a difference between a triplet excitationenergy of the host material and a singlet excitation energy of the hostmaterial is greater than 0.2 eV.
 18. The light-emitting elementaccording to claim 17, wherein the difference between the tripletexcitation energy of the host material and the singlet excitation energyof the host material is greater than or equal to 0.5 eV.
 19. Thelight-emitting element according to claim 17, wherein the first materialis a substance including a diazine skeleton or a triazine skeleton. 20.The light-emitting element according to claim 17, wherein the firstmaterial is a substance including a pyrazine skeleton or a pyrimidineskeleton.
 21. The light-emitting element according to claim 17, whereinthe first material is a substance including a diazine skeleton or atriazine skeleton.